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SCIENCE, TEXTBOOK, , FOR, , 2019-2020, , CLASS IX

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ISBN 81-7450-492-3, First Edition, February 2006 Phalguna 1927, Reprinted, November 2006, November 2007, January 2009, December 2009, November 2010, December 2011, October 2012, October 2013, December 2014, December 2015, February 2017, December 2017, December 2018, , Kartika 1928, Kartika 1929, Magha 1930, Pausa 1931, Kartika 1932, Pausa 1933, Asvina 1934, Asvina 1935, Agrahayana 1936, Agrahayana 1937, Phalguna 1938, Pausa 1939, Pausa 1940, , ALL RIGHTS RESERVED, q, , No part of this publication may be reproduced, stored in a retrieval system or, transmitted, in any form or by any means, electronic, mechanical, photocopying,, recording or otherwise without the prior permission of the publisher., , q, , This book is sold subject to the condition that it shall not, by way of trade, be lent, resold, hired out or otherwise disposed of without the publisher’s consent, in any form, of binding or cover other than that in which it is published., , q, , The correct price of this publication is the price printed on this page, Any revised, price indicated by a rubber stamp or by a sticker or by any other means is incorrect, and should be unacceptable., , OFFICES OF THE PUBLICATION DIVISION, NCERT, NCERT Campus, Sri Aurobindo Marg, New Delhi 110 016, , PD 650T BS, © National Council of Educational, Research and Training, 2006, , ` 150.00, , Printed on 80 GSM paper with NCERT, watermark, , Phone : 011-26562708, , 108, 100 Feet Road, Hosdakere Halli Extension, Banashankari III Stage, Bengaluru 560 085, , Phone : 080-26725740, , Navjivan Trust Building, P.O.Navjivan, Ahmedabad 380 014, , Phone : 079-27541446, , CWC Campus, Opp. Dhankal Bus Stop, Panihati, Kolkata 700 114, , Phone : 033-25530454, , CWC Complex, Maligaon, Guwahati 781 021, , Phone : 0361-2674869, , Publication Team, Head, Publication, Division, , : M. Siraj Anwar, , Chief Editor, , : Shveta Uppal, , Chief Business, Manager, , : Gautam Ganguly, , Chief Production, Officer, , : Arun Chitkara, , Production Assistant, , : Rajesh Pippal, , Published at the Publication Division, by the Secretary, National Council of, Educational Research and Training,, Sri Aurobindo Marg, New Delhi 110 016, and printed at Dee Kay Printers, 5/34, Kirti, Nagar, Industrial Area, New Delhi-110 015, , Cover, Nidhi Wadhwa, Layout and Illustrations, Digital Expressions, , 2019-2020

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FOREWORD, The National Curriculum Framework (NCF), 2005, recommends that children’s, life at school must be linked to their life outside the school. This principle, marks a departure from the legacy of bookish learning which continues to shape, our system and causes a gap between the school, home and community. The, syllabi and textbooks developed on the basis of NCF signify an attempt to, implement this basic idea. They also attempt to discourage rote learning and, the maintenance of sharp boundaries between different subject areas. We hope, these measures will take us significantly further in the direction of a childcentred system of education outlined in the National Policy on Education (1986)., The success of this effort depends on the steps that school principals and, teachers will take to encourage children to reflect on their own learning and to, pursue imaginative activities and questions. We must recognise that, given, space, time and freedom, children generate new knowledge by engaging with, the information passed on to them by adults. Treating the prescribed textbook, as the sole basis of examination is one of the key reasons why other resources, and sites of learning are ignored. Inculcating creativity and initiative is possible, if we perceive and treat children as participants in learning, not as receivers, of a fixed body of knowledge., These aims imply considerable change in school routines and mode of, functioning. Flexibility in the daily time-table is as necessary as rigour in, implementing the annual calendar so that the required number of teaching, days are actually devoted to teaching. The methods used for teaching and, evaluation will also determine how effective this textbook proves for making, children’s life at school a happy experience, rather than a source of stress or, boredom. Syllabus designers have tried to address the problem of curricular, burden by restructuring and reorienting knowledge at different stages with, greater consideration for child psychology and the time available for teaching., The textbook attempts to enhance this endeavour by giving higher priority and, , 2019-2020

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space to opportunities for contemplation and wondering, discussion in small, groups, and activities requiring hands-on experience., The National Council of Educational Research and Training (NCER T), appreciates the hard work done by the textbook development team responsible, for this book. We wish to thank the Chairman of the advisory group in science, and mathematics, Professor J.V. Narlikar and the Chief Advisor for this book,, Professor Rupamanjari Ghosh, School of Physical Sciences, Jawaharlal Nehru, University, New Delhi, for guiding the work of this committee. Several teachers, contributed to the development of this textbook; we are grateful to them and, their principals for making this possible. We are indebted to the institutions, and organisations which have generously permitted us to draw upon their, resources, material and personnel. We are especially grateful to the members, of the National Monitoring Committee, appointed by the Department of Secondary, and Higher Education, Ministry of Human Resource Development under the, Chairmanship of Professor Mrinal Miri and Professor G.P. Deshpande, for their, valuable time and contribution. As an organisation committed to systemic, reform and continuous improvement in the quality of its products, NCER T, welcomes comments and suggestions which will enable us to undertake further, revision and refinement., , Director, New Delhi, , National Council of Educational, Research and Training, , 20 December 2005, , (iv), , 2019-2020

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CONTENTS, FOREWORD, , iii, , Chapter 1, , MATTER IN OUR SURROUNDINGS, , 1, , Chapter 2, , IS MATTER AROUND US PURE?, , 14, , Chapter 3, , ATOMS AND MOLECULES, , 31, , Chapter 4, , STRUCTURE OF THE ATOM, , 46, , Chapter 5, , THE FUNDAMENTAL UNIT OF LIFE, , 57, , Chapter 6, , TISSUES, , 68, , Chapter 7, , DIVERSITY IN LIVING ORGANISMS, , 80, , Chapter 8, , MOTION, , 98, , Chapter 9, , FORCE AND LAWS OF MOTION, , 114, , Chapter 10, , GRAVITATION, , 131, , Chapter 11, , WORK, , Chapter 12, , SOUND, , Chapter 13, , WHY, , Chapter 14, , NATURAL RESOURCES, , 189, , Chapter 15, , IMPROVEMENT IN FOOD RESOURCES, , 203, , AND, , ENERGY, , DO WE, , 146, 160, , FALL ILL?, , ANSWERS, , 176, , 216 – 218, , 2019-2020

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2019-2020

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2019-2020

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THE CONSTITUTION OF, INDIA, PREAMBLE, WE, THE PEOPLE OF INDIA, having, solemnly resolved to constitute India into a, 1, [SOVEREIGN SOCIALIST SECULAR, DEMOCRATIC REPUBLIC] and to secure, to all its citizens :, JUSTICE,, political;, , social,, , economic, , and, , LIBERTY of thought, expression, belief,, faith and worship;, EQUALITY of status and of opportunity;, and to promote among them all, FRATERNITY assuring the dignity of, the individual and the 2[unity and, integrity of the Nation];, IN OUR CONSTITUENT ASSEMBLY, this twenty-sixth day of November, 1949 do, HEREBY ADOPT, ENACT AND GIVE TO, OURSELVES THIS CONSTITUTION., 1., 2., , Subs. by the Constitution (Forty-second Amendment) Act, 1976, Sec.2,, for "Sovereign Democratic Republic" (w.e.f. 3.1.1977), Subs. by the Constitution (Forty-second Amendment) Act, 1976, Sec.2,, for "Unity of the Nation" (w.e.f. 3.1.1977), , 2019-2020

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Chapter 1, MATTER, , IN, , OUR SURROUNDINGS, , As we look at our surroundings, we see a large, variety of things with different shapes, sizes, and textures. Everything in this universe is, made up of material which scientists have, named “matter”. The air we breathe, the food, we eat, stones, clouds, stars, plants and, animals, even a small drop of water or a, particle of sand – every thing is matter. We, can also see as we look around that all the, things mentioned above occupy space and, have mass. In other words, they have both, mass* and volume**., Since early times, human beings have, been trying to understand their surroundings., Early Indian philosophers classified matter in, the form of five basic elements – the, “Panch Tatva”– air, earth, fire, sky and water., According to them everything, living or nonliving, was made up of these five basic, elements. Ancient Greek philosophers had, arrived at a similar classification of matter., Modern day scientists have evolved two, types of classification of matter based on their, physical properties and chemical nature., In this chapter we shall learn about, matter based on its physical properties., Chemical aspects of matter will be taken up, in subsequent chapters., , 1.1 Physical Nature of Matter, 1.1.1 MATTER IS MADE UP OF PARTICLES, For a long time, two schools of thought prevailed, regarding the nature of matter. One school, believed matter to be continuous like a block, of wood, whereas, the other thought that matter, was made up of particles like sand. Let us, perform an activity to decide about the nature, of matter – is it continuous or particulate?, , Activity ______________ 1.1, •, •, •, •, •, •, •, , Take a 100 mL beaker., Fill half the beaker with water and, mark the level of water., Dissolve some salt/ sugar with the help, of a glass rod., Observe any change in water level., What do you think has happened to, the salt?, Where does it disappear?, Does the level of water change?, , In order to answer these questions we, need to use the idea that matter is made up, of particles. What was there in the spoon, salt, or sugar, has now spread throughout water., This is illustrated in Fig. 1.1., , Fig. 1.1: When we dissolve salt in water, the particles, of salt get into the spaces between particles, of water., , 1.1.2 HOW, , SMALL ARE THESE PARTICLES, , OF MATTER?, , Activity ______________ 1.2, •, , T ake 2-3 crystals of potassium, permanganate and dissolve them in, 100 mL of water., , * The SI unit of mass is kilogram (kg)., ** The SI unit of volume is cubic metre (m3). The common unit of measuring volume is, litre (L) such that 1L = 1 dm3 , 1L = 1000 mL, 1 mL = 1 cm3., , 2019-2020

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•, , Take out approximately 10 mL of this, solution and put it into 90 mL of clear, water., Take out 10 mL of this solution and, put it into another 90 mL of clear water., Keep diluting the solution like this 5 to, 8 times., Is the water still coloured ?, , •, •, •, , 1.2.2 P ARTICLES, , OF, , MATTER, , ARE, , CONTINUOUSLY MOVING, , Activity ______________ 1.3, •, , •, , •, , Put an unlit incense stick in a corner, of your class. How close do you have to, go near it so as to get its smell?, Now light the incense stick. What, happens? Do you get the smell sitting, at a distance?, Record your observations., , Activity ______________ 1.4, Fig. 1.2: Estimating how small are the particles of, matter. With every dilution, though the colour, becomes light, it is still visible., , This experiment shows that just a few, crystals of potassium permanganate can, colour a large volume of water (about, 1000 L). So we conclude that there must be, millions of tiny particles in just one crystal, of potassium permanganate, which keep on, dividing themselves into smaller and smaller, particles., The same activity can be done using, 2 mL of Dettol instead of potassium, permanganate. The smell can be detected, even on repeated dilution., , •, •, , •, •, •, •, •, , Take two glasses/beakers filled with, water., Put a drop of blue or red ink slowly, and carefully along the sides of the first, beaker and honey in the same way in, the second beaker., Leave them undisturbed in your house, or in a corner of the class., Record your observations., What do you observe immediately after, adding the ink drop?, What do you observe immediately after, adding a drop of honey?, How many hours or days does it take, for the colour of ink to spread evenly, throughout the water?, , Activity ______________ 1.5, , The particles of matter are very small –, they are small beyond our imagination!!!!, , •, , 1.2 Characteristics of Particles of, Matter, 1.2.1 PARTICLES OF MATTER HAVE SPACE, •, , BETWEEN THEM, , In activities 1.1 and 1.2 we saw that particles, of sugar, salt, Dettol, or potassium, permanganate got evenly distributed in water., Similarly, when we make tea, coffee or, lemonade (nimbu paani ), particles of one type, of matter get into the spaces between particles, of the other. This shows that there is enough, space between particles of matter., , •, •, •, , Drop a crystal of copper sulphate or, potassium permanganate into a glass, of hot water and another containing, cold water. Do not stir the solution., Allow the crystals to settle at the, bottom., What do you observe just above the, solid crystal in the glass?, What happens as time passes?, What does this suggest about the, particles of solid and liquid?, Does the rate of mixing change with, temperature? Why and how?, , From the above three activities (1.3, 1.4 and, 1.5), we can conclude the following:, , 2, , SCIENCE, , 2019-2020

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•, , Particles of matter are continuously, moving, that is, they possess what we call, the kinetic energy. As the temperature rises,, particles move faster. So, we can say that with, increase in temperature the kinetic energy of, the particles also increases., In the above three activities we observe, that particles of matter intermix on their own, with each other. They do so by getting into, the spaces between the particles. This, intermixing of particles of two different types, of matter on their own is called diffusion. We, also observe that on heating, diffusion, becomes faster. Why does this happen?, , 1.2.3 P ARTICLES, , Activity ______________ 1.7, •, •, •, , Activity ______________ 1.8, •, , Activity ______________ 1.6, , •, , •, , •, , Play this game in the field— make four, groups and form human chains as, suggested:, The first group should hold each, other from the back and lock arms, like Idu-Mishmi dancers (Fig. 1.3)., , •, , •, , •, , MATTER, , The second group should hold hands, to form a human chain., The third group should form a chain, by touching each other with only their, finger tips., Now, the fourth group of students, should run around and try to break the, three human chains one by one into, as many small groups as possible., Which group was the easiest to break?, Why?, IN, , Take some water in a container, try, cutting the surface of water with your, fingers., Were you able to cut the surface of, water?, What could be the reason behind the, surface of water remaining together?, , The above three activities (1.6, 1.7 and 1.8), suggest that particles of matter have force, acting between them. This force keeps the, particles together. The strength of this force of, attraction varies from one kind of matter to, another., , Q, , Fig. 1.3, , •, , Take an iron nail, a piece of chalk and, a rubber band., Try breaking them by hammering,, cutting or stretching., In which of the above three, substances do you think the particles, are held together with greater force?, , OF MATTER ATTRACT, , EACH OTHER, , •, , If we consider each student as a, particle of matter, then in which group, the particles held each other with the, maximum force?, , uestions, 1., , Which of the following are, matter?, Chair, air, love, smell, hate,, almonds, thought, cold, lemon, water, smell of perfume., 2. Give reasons for the following, observation:, The smell of hot sizzling food, reaches you several metres, away, but to get the smell from, cold food you have to go close., 3. A diver is able to cut through, water in a swimming pool. Which, property of matter does this, observation show?, 4. What are the characteristics of, the particles of matter?, 3, , OUR SURROUNDING S, , 2019-2020

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1.3 States of Matter, , the force is removed. If excessive force is, applied, it breaks., The shape of each individual sugar or, salt crystal remains fixed, whether we, take it in our hand, put it in a plate or in, a jar., A sponge has minute holes, in which, air is trapped, when we press it, the air, is expelled out and we are able to, compress it., , Observe different types of matter around you., What are its different states? We can see that, matter around us exists in three different, states– solid, liquid and gas. These states of, matter arise due to the variation in the, characteristics of the particles of matter., Now, let us study about the properties of, these three states of matter in detail., , •, , 1.3.1 THE SOLID STATE, , 1.3.2 THE LIQUID STATE, , •, , Activity _____________ 1.9, , Activity _____________ 1.10, , •, , •, , Collect the following articles— a pen,, a book, a needle and a piece of wooden, stick., Sketch the shape of the above articles, in your notebook by moving a pencil, around them., Do all these have a definite shape,, distinct boundaries and a fixed volume?, What happens if they are hammered,, pulled or dropped?, Are these capable of diffusing into each, other?, Try compressing them by applying, force. Are you able to compress them?, , •, , •, •, •, •, , •, •, , •, , All the above are examples of solids. We, can observe that all these have a definite, shape, distinct boundaries and fixed volumes,, that is, have negligible compressibility. Solids, have a tendency to maintain their shape when, subjected to outside force. Solids may break, under force but it is difficult to change their, shape, so they are rigid., Consider the following:, (a) What about a rubber band, can it, change its shape on stretching? Is it, a solid?, (b) What about sugar and salt? When, kept in different jars these take the, shape of the jar. Are they solid?, (c) What about a sponge? It is a solid, yet we are able to compress it. Why?, All the above are solids as:, •, A rubber band changes shape under, force and regains the same shape when, , •, , Collect the following:, (a) water, cooking oil, milk, juice, a, cold drink., (b) containers of different shapes. Put, a 50 mL mark on these containers, using a measuring cylinder from, the laboratory., What will happen if these liquids are, spilt on the floor?, Measure 50 mL of any one liquid and, transfer it into different containers one, by one. Does the volume remain the, same?, Does the shape of the liquid remain the, same ?, When you pour the liquid from one, container into another, does it flow, easily?, , We observe that liquids have no fixed, shape but have a fixed volume. They take up, the shape of the container in which they are, kept. Liquids flow and change shape, so they, are not rigid but can be called fluid., Refer to activities 1.4 and 1.5 where we, saw that solids and liquids can diffuse into, liquids. The gases from the atmosphere, diffuse and dissolve in water. These gases,, especially oxygen and carbon dioxide, are, essential for the survival of aquatic animals, and plants., All living creatures need to breathe for, survival. The aquatic animals can breathe, under water due to the presence of dissolved, oxygen in water. Thus, we may conclude that, solids, liquids and gases can diffuse into, liquids. The rate of diffusion of liquids is, , 4, , SCIENCE, , 2019-2020

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higher than that of solids. This is due to the, fact that in the liquid state, particles move, freely and have greater space between each, other as compared to particles in the solid, state., , 1.3.3 THE GASEOUS STATE, Have you ever observed a balloon seller filling, a large number of balloons from a single, cylinder of gas? Enquire from him how many, balloons is he able to fill from one cylinder., Ask him which gas does he have in the cylinder., , Activity _____________ 1.11, •, , •, •, , •, , •, , Take three 100 mL syringes and close, their nozzles by rubber corks, as, shown in Fig.1.4., Remove the pistons from all the, syringes., Leaving one syringe untouched, fill, water in the second and pieces of chalk, in the third., Insert the pistons back into the, syringes. You may apply some vaseline, on the pistons before inserting them, into the syringes for their smooth, movement., Now, try to compress the content by, pushing the piston in each syringe., , We have observed that gases are highly, compressible as compared to solids and, liquids. The liquefied petroleum gas (LPG), cylinder that we get in our home for cooking, or the oxygen supplied to hospitals in, cylinders is compressed gas. Compressed, natural gas (CNG) is used as fuel these days, in vehicles. Due to its high compressibility,, large volumes of a gas can be compressed, into a small cylinder and transported easily., We come to know of what is being cooked, in the kitchen without even entering there,, by the smell that reaches our nostrils. How, does this smell reach us? The particles of the, aroma of food mix with the particles of air, spread from the kitchen, reach us and even, farther away. The smell of hot cooked food, reaches us in seconds; compare this with the, rate of diffusion of solids and liquids. Due to, high speed of particles and large space, between them, gases show the property of, diffusing very fast into other gases., In the gaseous state, the particles move, about randomly at high speed. Due to this, random movement, the particles hit each, other and also the walls of the container. The, pressure exerted by the gas is because of this, force exerted by gas particles per unit area, on the walls of the container., , Fig. 1.4, , •, •, , MATTER, , What do you observe? In which case, was the piston easily pushed in?, What do you infer from your, observations?, IN, , Fig.1.5: a, b and c show the magnified schematic, pictures of the three states of matter. The, motion of the particles can be seen and, compared in the three states of matter., 5, , OUR SURROUNDING S, , 2019-2020

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Q, , 1.4.1 EFFECT OF CHANGE OF TEMPERATURE, , uestions, 1. The mass per unit volume of a, substance is called density., (density = mass/volume)., Arrange the following in order of, increasing density – air, exhaust, from chimneys, honey, water,, chalk, cotton and iron., 2. (a) Tabulate the differences in, the characterisitcs of states, of matter., (b) Comment upon the following:, rigidity, compressibility,, fluidity, filling a gas, container, shape, kinetic, energy and density., 3. Give reasons, (a) A gas fills completely the, vessel in which it is kept., (b) A gas exerts pressure on the, walls of the container., (c) A wooden table should be, called a solid., (d) We can easily move our hand, in air but to do the same, through a solid block of wood, we need a karate expert., 4. Liquids generally have lower, density as compared to solids., But you must have observed that, ice floats on water. Find out why., , Activity _____________ 1.12, •, , Take about 150 g of ice in a beaker and, suspend a laboratory thermometer so, that its bulb is in contact with the ice,, as in Fig. 1.6., , (a), , 1.4 Can Matter Change its State?, We all know from our observation that water, can exist in three states of matter–, •, •, •, , solid, as ice,, liquid, as the familiar water, and, gas, as water vapour., , What happens inside the matter during, this change of state? What happens to the, particles of matter during the change of, states? How does this change of state take, place? We need answers to these questions,, isn’t it?, , (b), , Fig. 1.6: (a) Conversion of ice to water, (b) conversion, of water to water vapour, , 6, , SCIENCE, , 2019-2020

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•, •, , Start heating the beaker on a low flame., Note the temperature when the ice, starts melting., Note the temperature when all the ice, has converted into water., Record your observations for this, conversion of solid to liquid state., Now, put a glass rod in the beaker and, heat while stirring till the water starts, boiling., Keep a careful eye on the thermometer, reading till most of the water has, vaporised., Record your observations for the, conversion of water in the liquid state, to the gaseous state., , •, •, •, , •, , •, , On increasing the temperature of solids,, the kinetic energy of the particles increases., Due to the increase in kinetic energy, the, particles start vibrating with greater speed., The energy supplied by heat overcomes the, forces of attraction between the particles. The, particles leave their fixed positions and start, moving more freely. A stage is reached when, the solid melts and is converted to a liquid., The minimum temperature at which a solid, melts to become a liquid at the atmospheric, pressure is called its melting point., The melting point of a solid is an indication, of the strength of the force of attraction, between its particles., The melting point of ice is 273.15 K*. The, process of melting, that is, change of solid, state into liquid state is also known as fusion., When a solid melts, its temperature, remains the same, so where does the heat, energy go?, You must have observed, during the, experiment of melting, that the temperature, of the system does not change after the, melting point is reached, till all the ice melts., This happens even though we continue to, heat the beaker, that is, we continue to supply, heat. This heat gets used up in changing the, , state by overcoming the forces of attraction, between the particles. As this heat energy is, absorbed by ice without showing any rise in, temperature, it is considered that it gets, hidden into the contents of the beaker and is, known as the latent heat. The word latent, means hidden. The amount of heat energy, that is required to change 1 kg of a solid into, liquid at atmospheric pressure at its melting, point is known as the latent heat of fusion., So, particles in water at 00 C (273 K) have, more energy as compared to particles in ice, at the same temperature., When we supply heat energy to water,, particles start moving even faster. At a certain, temperature, a point is reached when the, particles have enough energy to break free, from the forces of attraction of each other. At, this temperature the liquid starts changing, into gas. The temperature at which a liquid, starts boiling at the atmospheric pressure is, known as its boiling point. Boiling is a bulk, phenomenon. Particles from the bulk of the, liquid gain enough energy to change into the, vapour state., For water this temperature is 373 K, (100 0C = 273 + 100 = 373 K)., Can you define the latent heat of, vaporisation? Do it in the same way as we, have defined the latent heat of fusion., Particles in steam, that is, water vapour at, 373 K (1000 C) have more energy than water, at the same temperature. This is because, particles in steam have absorbed extra energy, in the form of latent heat of vaporisation., , So, we infer that the state of matter can, be changed into another state by changing, the temperature., We have learnt that substances around, us change state from solid to liquid and from, liquid to gas on application of heat. But there, , *Note: Kelvin is the SI unit of temperature, 00 C =273.15 K. For convenience, we take 00 C = 273 K after, rounding off the decimal. To change a temperature on the Kelvin scale to the Celsius scale you, have to subtract 273 from the given temperature, and to convert a temperature on the Celsius, scale to the Kelvin scale you have to add 273 to the given temperature., MATTER, , IN, , 7, , OUR SURROUNDING S, , 2019-2020

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are some that change directly from solid state, to gaseous state and vice versa without, changing into the liquid state., , Activity _____________ 1.13, •, , enclosed in a cylinder? Will the particles come, closer? Do you think that increasing or, decreasing the pressure can change the state, of matter?, , Take some camphor or ammonium, chloride. Crush it and put it in a china, dish., Put an inverted funnel over the china, dish., Put a cotton plug on the stem of the, funnel, as shown in Fig. 1.7., , •, •, , Fig. 1.8: By applying pressure, particles of matter can, be brought close together., , Fig. 1.7: Sublimation of ammonium chloride, , •, •, , Now, heat slowly and observe., What do you infer from the above, activity?, , A change of state directly from solid to gas, without changing into liquid state is called, sublimation and the direct change of gas to, solid without changing into liquid is called, deposition., , Applying pressure and reducing, temperature can liquefy gases., Have you heard of solid carbon dioxide, (CO2)? It is stored under high pressure. Solid, CO2 gets converted directly to gaseous state, on decrease of pressure to 1 atmosphere*, without coming into liquid state. This is the, reason that solid carbon dioxide is also known, as dry ice., Thus, we can say that pressure and, temperature deter mine the state of a, substance, whether it will be solid, liquid or, gas., , 1.4.2 EFFECT OF CHANGE OF PRESSURE, We have already learnt that the difference in, various states of matter is due to the, difference in the distances between the, constituent particles. What will happen when, we start putting pressure and compress a gas, , Deposition, , Fig. 1.9: Interconversion of the three states of matter, * atmosphere (atm) is a unit of measuring pressure exerted by a gas. The unit of pressure is Pascal (Pa):, 1 atmosphere = 1.01 × 105 Pa. The pressure of air in atmosphere is called atmospheric pressure. The, atmospheric pressure at sea level is 1 atmosphere, and is taken as the normal atmospheric pressure., 8, , SCIENCE, , 2019-2020

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Q, , uestions, 1. Convert, the, following, temperature to celsius scale:, a. 300 K, b. 573 K., 2. What is the physical state of, water at:, a. 250ºC, b. 100ºC ?, 3. For any substance, why does the, temperature remain constant, during the change of state?, 4. Suggest a method to liquefy, atmospheric gases., , 1.5 Evaporation, Do we always need to heat or change pressure, for changing the state of matter? Can you, quote some examples from everyday life where, change of state from liquid to vapour takes, place without the liquid reaching the boiling, point? Water, when left uncovered, slowly, changes into vapour. Wet clothes dry up., What happens to water in the above two, examples?, We know that particles of matter are, always moving and are never at rest. At a, given temperature in any gas, liquid or solid,, there are particles with different amounts of, kinetic energy. In the case of liquids, a small, fraction of particles at the surface, having, higher kinetic energy, is able to break away, from the forces of attraction of other particles, and gets converted into vapour. This, phenomenon of change of a liquid into, vapours at any temperature below its boiling, point is called evaporation., , 1.5.1 FACTORS AFFECTING EVAPORATION, Let us understand this with an activity., , •, •, •, , •, , You must have observed that the rate of, evaporation increases with–, •, an increase of surface area:, We know that evaporation is a surface, phenomenon. If the surface area is, increased, the rate of evaporation, increases. For example, while putting, clothes for drying up we spread them out., •, an increase of temperature:, With the increase of temperature, more, number of particles get enough kinetic, energy to go into the vapour state., •, a decrease in humidity:, Humidity is the amount of water vapour, present in air. The air around us cannot, hold more than a definite amount of, water vapour at a given temperature. If, the amount of water in air is already high,, the rate of evaporation decreases., •, an increase in wind speed:, It is a common observation that clothes, dry faster on a windy day. With the, increase in wind speed, the particles of, water vapour move away with the wind,, decreasing the amount of water vapour, in the surrounding., , 1.5.2 HOW, , •, , •, MATTER, , Take 5 mL of water in a test tube and, keep it near a window or under a fan., Take 5 mL of water in an open china, dish and keep it near a window or, under a fan., Take 5 mL of water in an open china, IN, , DOES EVAPORATION CAUSE, , COOLING?, , Activity _____________ 1.14, •, , dish and keep it inside a cupboard or, on a shelf in your class., Record the room temperature., Record the time or days taken for the, evaporation process in the above cases., Repeat the above three steps of activity, on a rainy day and recor d your, observations., What do you infer about the effect of, temperature, surface area and wind, velocity (speed) on evaporation?, , In an open vessel, the liquid keeps on, evaporating. The particles of liquid absorb, energy from the surrounding to regain the, energy lost during evaporation. This, absorption of energy from the surroundings, make the surroundings cold., 9, , OUR SURROUNDING S, , 2019-2020

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What happens when you pour some, acetone (nail polish remover) on your palm?, The particles gain energy from your palm or, surroundings and evaporate causing the, palm to feel cool., After a hot sunny day, people sprinkle, water on the roof or open ground because, the large latent heat of vaporisation of water, helps to cool the hot surface., Can you cite some more examples from, daily life where we can feel the effect of cooling, due to evaporation?, Why should we wear cotton clothes in, summer?, During summer, we perspire more, because of the mechanism of our body which, keeps us cool. We know that during, evaporation, the particles at the surface of, the liquid gain energy from the surroundings, or body surface and change into vapour. The, heat energy equal to the latent heat of, vaporisation is absorbed from the body, leaving the body cool. Cotton, being a good, absorber of water helps in absorbing the, sweat and exposing it to the atmosphere for, easy evaporation., , Why do we see water droplets on the outer, surface of a glass containing ice-cold, water?, Let us take some ice-cold water in a, tumbler. Soon we will see water droplets on, the outer surface of the tumbler. The water, vapour present in air, on coming in contact, with the cold glass of water, loses energy and, gets converted to liquid state, which we see, as water droplets., , Q, , uestions, 1. Why does a desert cooler cool, better on a hot dry day?, 2. How does the water kept in an, earthen pot (matka) become cool, during summer?, 3. Why does our palm feel cold, when we put some acetone or, petrol or perfume on it?, 4. Why are we able to sip hot tea or, milk faster from a saucer rather, than a cup?, 5. What type of clothes should we, wear in summer?, , Now scientists are talking of five states of matter: Solid, Liquid, Gas, Plasma and BoseEinstein Condensate., , More to know, , Plasma: The state consists of super energetic and super excited particles. These particles, are in the form of ionised gases. The fluorescent tube and neon sign bulbs consist of, plasma. Inside a neon sign bulb there is neon gas and inside a fluorescent tube there, is helium gas or some other gas. The gas gets ionised, that is, gets charged when, electrical energy flows through it. This charging up creates a plasma glowing inside, the tube or bulb. The plasma glows with a special colour depending on the nature of, gas. The Sun and the stars glow because of the presence of plasma in them. The plasma, is created in stars because of very high temperature., Bose-Einstein Condensate: In 1920, Indian physicist Satyendra Nath Bose had done, some calculations for a fifth state of matter. Building on his calculations, Albert Einstein, predicted a new state of matter – the Bose-Einstein, Condensate (BEC). In 2001, Eric A. Cornell, Wolfgang, Ketterle and Carl E. Wieman of USA received the Nobel, prize in physics for achieving “Bose-Einstein, condensation”. The BEC is formed by cooling a gas of, extremely low density, about one-hundred-thousandth, the density of normal air, to super low temperatures., You can log on to www.chem4kids.com to get more, S.N. Bose, Albert Einstein, information on these fourth and fifth states of matter., (1894-1974), (1879-1955), 10, , SCIENCE, , 2019-2020

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What, you have, learnt, , MATTER, , IN, , •, , Matter is made up of small particles., , •, , The matter around us exists in three states— solid, liquid, and gas., , •, , The forces of attraction between the particles are maximum in, solids, intermediate in liquids and minimum in gases., , •, , The spaces in between the constituent particles and kinetic, energy of the particles are minimum in the case of solids,, intermediate in liquids and maximum in gases., , •, , The arrangement of particles is most ordered in the case of, solids, in the case of liquids layers of particles can slip and, slide over each other while for gases, there is no order, particles, just move about randomly., , •, , The states of matter are inter-convertible. The state of matter, can be changed by changing temperature or pressure., , •, , Sublimation is the change of solid state directly to gaseous, state without going through liquid state., , •, , Deposition is the change of gaseous state directly to solid, state without going through liquid state., , •, , Boiling is a bulk phenomenon. Particles from the bulk, (whole) of the liquid change into vapour state., , •, , Evaporation is a surface phenomenon. Particles from the, surface gain enough energy to overcome the forces of attraction, present in the liquid and change into the vapour state., , •, , The rate of evaporation depends upon the surface area exposed, to the atmosphere, the temperature, the humidity and the, wind speed., , •, , Evaporation causes cooling., , •, , Latent heat of vaporisation is the heat energy required to change, 1 kg of a liquid to gas at atmospheric pressure at its boiling, point., , •, , Latent heat of fusion is the amount of heat energy required, to change 1 kg of solid into liquid at its melting point., 11, , OUR SURROUNDING S, , 2019-2020

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•, , Some measurable quantities and their units to remember:, Quantity, , Unit, , Symbol, , Temperature, Length, Mass, Weight, Volume, Density, Pressure, , kelvin, metre, kilogram, newton, cubic metre, kilogram per cubic metre, pascal, , K, m, kg, N, m3, kg m–3, , Pa, , Exercises, 1. Convert the following temperatures to the celsius scale., (a) 293 K, (b) 470 K., 2. Convert the following temperatures to the kelvin scale., (a) 25°C, (b) 373°C., 3. Give reason for the following observations., (a) Naphthalene balls disappear with time without leaving, any solid., (b) We can get the smell of perfume sitting several metres, away., 4. Arrange the following substances in increasing order of forces, of attraction between the particles— water, sugar, oxygen., 5. What is the physical state of water at—, (a) 25°C, (b) 0°C, (c) 100°C ?, 6. Give two reasons to justify—, (a) water at room temperature is a liquid., (b) an iron almirah is a solid at room temperature., 7. Why is ice at 273 K more effective in cooling than water at the, same temperature?, 8. What produces more severe burns, boiling water or steam?, 9. Name A,B,C,D,E and F in the following diagram showing, change in its state, , 12, , SCIENCE, , 2019-2020

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Group Activity, Prepare a model to demonstrate movement of particles in solids,, liquids and gases., For making this model you will need, •, A transparent jar, •, A big rubber balloon or piece of stretchable rubber sheet, •, A string, •, Few chick-peas or black gram or dry green peas., How to make?, •, Put the seeds in the jar., •, Sew the string to the centre of the rubber sheet and put some, tape to keep it tied securely., •, Stretch and tie the rubber sheet on the mouth of the jar., •, Your model is ready. Now run your fingers up and down the, string by first tugging at it slowly and then rapidly., , Fig. 1.10: A model for converting of solid to liquid and liquid to gas., , MATTER, , IN, , 13, , OUR SURROUNDING S, , 2019-2020

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Chapter 2, IS MATTER AROUND US PURE?, How do we judge whether milk, ghee, butter,, salt, spices, mineral water or juice that we, buy from the market are pure?, , Fig. 2.1: Some consumable items, , evaporation. However, sodium chloride is itself, a pure substance and cannot be separated by, physical process into its chemical constituents., Similarly, sugar is a substance which contains, only one kind of pure matter and its, composition is the same throughout., Soft drink and soil are not single pure, substances. Whatever the source of a, pure substance may be, it will always have, the same characteristic properties., Therefore, we can say that a mixture, contains more than one pure substance., , 2.1.1 TYPES OF MIXTURES, , Have you ever noticed the word ‘pure’, written on the packs of these consumables?, For a common person pure means having no, adulteration. But, for a scientist all these things, are actually mixtures of different substances, and hence not pure. For example, milk is, actually a mixture of water, fat, proteins etc., When a scientist says that something is pure,, it means that all the constituent particles of, that substance are the same in their chemical, nature. A pure substance consists of a single, type of particles. In other words, a substance, is a pure single form of matter., As we look around, we can see that most, of the matter around us exist as mixtures of, two or more pure components, for example,, sea water, minerals, soil etc. are all mixtures., , 2.1 What is a Mixture?, , Depending upon the nature of the components, that form a mixture, we can have different, types of mixtures., , Activity ______________ 2.1, •, •, , •, , •, •, , Mixtures are constituted by more than one, kind of pure form of matter. We know that, dissolved sodium chloride can be separated, from water by the physical process of, , 2019-2020, , Let us divide the class into groups A,, B, C and D., Group A takes a beaker containing, 50 mL of water and one spatula full of, copper sulphate powder. Group B takes, 50 mL of water and two spatula full of, copper sulphate powder in a beaker., Groups C and D can take different, amounts of copper sulphate and, potassium permanganate or common, salt (sodium chloride) and mix the given, components to form a mixture., Report the observations on the, uniformity in colour and texture., Groups A and B have obtained a, mixtur e which has a unifor m, composition throughout. Such, mixtures are called homogeneous, mixtures or solutions. Some other, examples of such mixtures are: (i) salt, dissolved in water and (ii) sugar

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Activity ______________ 2.2, •, •, , •, , •, , •, , •, , •, •, •, , Let us again divide the class into four, groups – A, B, C and D., Distribute the following samples to, each group:, − Few crystals of copper sulphate to, group A., − One spatula full of copper, sulphate to group B., − Chalk powder or wheat flour to, group C., − Few dr ops of milk or ink to, group D., Each group should add the given, sample in water and stir properly using, a glass rod. Are the particles in the, mixture visible?, Direct a beam of light from a torch, through the beaker containing the, mixture and observe from the front., Was the path of the beam of light, visible?, Leave the mixtures undisturbed for a, few minutes (and set up the filtration, apparatus in the meantime). Is the, mixture stable or do the particles begin, to settle after some time?, Filter the mixture. Is there any residue, on the filter paper?, Discuss the results and for m an, opinion., Groups A and B have got a solution., Group C has got a suspension., Group D has got a colloidal solution., , Fig. 2.2: Filtration, , Now, we shall learn about solutions,, suspensions and colloidal solutions in the, following sections., , Q, , uestions, 1. What is meant by a substance?, 2. List the points of differences, between homogeneous and, heterogeneous mixtures., , 2.2 What is a Solution?, A solution is a homogeneous mixture of two, or more substances. You come across various, types of solutions in your daily life. Lemonade,, soda water etc. are all examples of solutions., Usually we think of a solution as a liquid that, contains either a solid, liquid or a gas, dissolved in it. But, we can also have solid, solutions (alloys) and gaseous solutions (air)., In a solution there is homogeneity at the, particle level. For example, lemonade tastes the, same throughout. This shows that particles of, sugar or salt are evenly distributed in the, solution., , More to know, , •, , dissolved in water. Compar e the, colour of the solutions of the two, groups. Though both the groups have, obtained copper sulphate solution but, the intensity of colour of the solutions, is different. This shows that a, homogeneous mixture can have a, variable composition., Gr oups C and D have obtained, mixtures, which contain physically, distinct parts and have non-uniform, compositions. Such mixtures are called, heterogeneous mixtures. Mixtures of, sodium chloride and iron filings, salt, and sulphur, and oil and water are, examples of heterogeneous mixtures., , Alloys: Alloys are mixtures of two or, more metals or a metal and a non-metal, and cannot be separated into their, components by physical methods. But, still, an alloy is considered as a mixture, because it shows the properties of its, constituents and can have variable, composition. For example, brass is a, mixture of approximately 30% zinc and, 70% copper., 15, , IS MATTER AROUND US PURE?, , 2019-2020

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A solution has a solvent and a solute as its, components. The component of the solution, that dissolves the other component in it, (usually the component present in larger, amount) is called the solvent. The component, of the solution that is dissolved in the solvent, (usually present in lesser quantity) is called, the solute., , Activity ______________ 2.3, , Examples:, (i) A solution of sugar in water is a solid, in liquid solution. In this solution,, sugar is the solute and water is the, solvent., (ii) A solution of iodine in alcohol known, as ‘tincture of iodine’, has iodine (solid), as the solute and alcohol (liquid) as, the solvent., (iii) Aerated drinks like soda water etc., are, gas in liquid solutions. These contain, carbon dioxide (gas) as solute and, water (liquid) as solvent., (iv) Air is a mixture of gas in gas. Air is a, homogeneous mixture of a number of, gases. Its two main constituents are:, oxygen (21%) and nitrogen (78%). The, other gases are present in very small, quantities., , Properties of a solution, •, •, , •, , •, , proportion of the solute and solvent can be, varied. Depending upon the amount of solute, present in a solution, it can be called a dilute,, concentrated or a saturated solution. Dilute, and concentrated are comparative terms. In, activity 2.2, the solution obtained by group A, is dilute as compared to that obtained by, group B., , A solution is a homogeneous mixture., The particles of a solution are smaller, than 1 nm (10-9 metre) in diameter. So,, they cannot be seen by naked eyes., Because of very small particle size, they, do not scatter a beam of light passing, through the solution. So, the path of, light is not visible in a solution., The solute particles cannot be, separated from the mixture by the, process of filtration. The solute particles, do not settle down when left undisturbed,, that is, a solution is stable., , 2.2.1 CONCENTRATION OF A SOLUTION, In activity 2.2, we observed that groups A and, B obtained different shades of solutions. So,, we understand that in a solution the relative, , •, •, , •, , •, , Take approximately 50 mL of water, each in two separate beakers., Add salt in one beaker and sugar or, barium chloride in the second beaker, with continuous stirring., When no more solute can be dissolved,, heat the contents of the beaker to raise, the temperature by about 5°C., Start adding the solute again., , Is the amount of salt and sugar or barium, chloride, that can be dissolved in water at a, given temperature, the same?, At any particular temperature, a solution, that has dissolved as much solute as it is, capable of dissolving, is said to be a saturated, solution. In other words, when no more solute, can be dissolved in a solution at a given, temperature, it is called a saturated solution., The amount of the solute present in the, saturated solution at this temperature is called, its solubility., If the amount of solute contained in a, solution is less than the saturation level, it is, called an unsaturated solution., What would happen if you were to take a, saturated solution at a certain temperature, and cool it slowly., We can infer from the above activity that, different substances in a given solvent have, different solubilities at the same temperature., The concentration of a solution is the amount, (mass or volume) of solute present in a given, amount (mass or volume) of solution., There are various ways of expressing the, concentration of a solution, but here we will, learn only three methods., (i) Mass by mass percentage of a solution, Mass of solute, =, ×100, Mass of solution, , 16, , SCIENCE, , 2019-2020

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(ii) Mass by volume percentage of a solution, Mass of solute, , =, , Volume of solution, , •, •, , ×100, , (iii) Volume by volume percentage of a, solution, Volume of solute, , =, , Volume of solution, , ×100, , Example 2.1 A solution contains 40 g of, common salt in 320 g of water., Calculate the concentration in terms of, mass by mass percentage of the, solution., Solution:, Mass of solute (salt), = 40 g, Mass of solvent (water) = 320 g, We know,, Mass of solution = Mass of solute +, Mass of solvent, = 40 g + 320 g, = 360 g, Mass percentage of solution, =, , =, , Mass of solute, Mass of solution, , ×100, , 40, × 100 =11.1%, 360, , 2.2.2 What is a suspension?, Non-homogeneous systems, like those, obtained by group C in activity 2.2, in which, solids are dispersed in liquids, are called, suspensions. A suspension is a heterogeneous, mixture in which the solute particles do not, dissolve but remain suspended throughout, the bulk of the medium. Particles of a, suspension are visible to the naked eye., , Properties of a Suspension, •, , •, , Suspension is a heterogeneous, mixture., , The particles of a suspension can be seen, by the naked eye., The particles of a suspension scatter a, beam of light passing through it and, make its path visible., The solute particles settle down when a, suspension is left undisturbed, that is,, a suspension is unstable. They can be, separated from the mixture by the, process of filtration. When the particles, settle down, the suspension breaks and, it does not scatter light any more., , 2.2.3 WHAT IS A COLLOIDAL SOLUTION?, The mixture obtained by group D in activity, 2.2 is called a colloid or a colloidal solution., The particles of a colloid are uniformly spread, throughout the solution. Due to the relatively, smaller size of particles, as compared to that of, a suspension, the mixture appears to be, homogeneous. But actually, a colloidal solution, is a heterogeneous mixture, for example, milk., Because of the small size of colloidal, particles, we cannot see them with naked eyes., But, these particles can easily scatter a beam, of visible light as observed in activity 2.2. This, scattering of a beam of light is called the, Tyndall effect after the name of the scientist, who discovered this effect., Tyndall effect can also be observed when a, fine beam of light enters a room through a small, hole. This happens due to the scattering of light, by the particles of dust and smoke in the air., , (a), , (b), , Fig. 2.3: (a) Solution of copper sulphate does not show, Tyndall effect, (b) mixture of water and milk, shows Tyndall effect., , 17, , IS MATTER AROUND US PURE?, , 2019-2020

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•, , Tyndall effect can be observed when, sunlight passes through the canopy of a dense, forest. In the forest, mist contains tiny droplets, of water, which act as particles of colloid, dispersed in air., , They cannot be separated from the, mixture by the process of filtration. But,, a special technique of separation known, as centrifugation (perform activity 2.5),, can be used to separate the colloidal, particles., , The components of a colloidal solution are, the dispersed phase and the dispersion, medium. The solute-like component or the, dispersed particles in a colloid form the, dispersed phase, and the component in which, the dispersed phase is suspended is known, as the dispersing medium. Colloids are, classified according to the state (solid, liquid, or gas) of the dispersing medium and the, dispersed phase. A few common examples are, given in Table 2.1. From this table you can, see that they are very common everyday life., Fig. 2.4: The Tyndall effect, , Properties of a colloid, •, •, , •, , •, , A colloid is a heterogeneous mixture., The size of particles of a colloid is too, small to be individually seen by naked, eyes., Colloids are big enough to scatter a, beam of light passing through it and, make its path visible., They do not settle down when left, undisturbed, that is, a colloid is quite, stable., , Q, , uestions, 1. Differentiate between homogeneous and heterogeneous mixtures, with examples., 2. How are sol, solution and, suspension different from each, other?, 3. To make a saturated solution,, 36 g of sodium chloride is dissolved, in 100 g of water at 293 K., Find its concentration at this, temperature., , Table 2.1: Common examples of colloids, Dispersed, phase, , Dispersing, Medium, , Type, , Example, , Liquid, , Gas, , Aerosol, , Fog, clouds, mist, , Solid, , Gas, , Aerosol, , Smoke, automobile exhaust, , Gas, , Liquid, , Foam, , Shaving cream, , Liquid, , Liquid, , Emulsion, , Milk, face cream, , Solid, , Liquid, , Sol, , Milk of magnesia, mud, , Gas, , Solid, , Foam, , Foam, rubber, sponge, pumice, , Liquid, , Solid, , Gel, , Jelly, cheese, butter, , Solid, , Solid, , Solid Sol, , Coloured gemstone, milky glass, , 18, , SCIENCE, , 2019-2020

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2.3 Separating the Components, of a Mixture, We have learnt that most of the natural, substances are not chemically pure. Different, methods of separation are used to get, individual components from a mixture., Separation makes it possible to study and, use the individual components of a mixture., Heterogeneous mixtures can be separated, into their respective constituents by simple, physical methods like handpicking, sieving,, filtration that we use in our day-to-day life., Sometimes special techniques have to be used, for the separation of the components of a, mixture., , 2.3.1 HOW, , ( DYE ), , FROM BLUE /, , BLACK INK?, , Activity ______________ 2.4, , •, •, , •, , •, , •, , What do you think has got evaporated, from the watch glass?, , •, , Is there a residue on the watch glass?, , •, , What is your interpretation? Is ink a, single substance (pure) or is it a, mixture?, , We find that ink is a mixture of a dye in, water. Thus, we can separate the volatile, component (solvent) from its non-volatile, solute by the method of evaporation., , 2.3.2 HOW, , CAN WE SEPARATE CREAM, , FROM MILK?, , CAN WE OBTAIN COLOURED, , COMPONENT, , •, •, , Now answer, , Now-a-days, we get full-cream, toned and, double-toned varieties of milk packed in polypacks or tetra packs in the market. These, varieties of milk contain different amounts, of fat., , Fill half a beaker with water., Put a watch glass on the mouth of the, beaker (Fig. 2.5)., Put few drops of ink on the watch glass., Now start heating the beaker. We do, not want to heat the ink directly. You, will see that evaporation is taking place, from the watch glass., Continue heating as the evaporation, goes on and stop heating when you do, not see any further change on the, watch glass., Observe carefully and record your, observations., , Activity ______________ 2.5, •, •, , •, , Take some full-cream milk in a test, tube., Centrifuge it by using a centrifuging, machine for two minutes. If a, centrifuging machine is not available, in the school, you can do this activity, at home by using a milk churner, used, in the kitchen., If you have a milk dairy nearby, visit it, and ask (i) how they separate cream, from milk and (ii) how they make, cheese (paneer) from milk., , Now answer, , Fig. 2.5: Evaporation, , •, , What do you observe on churning the, milk?, , •, , Explain how the separation of cream, from milk takes place., , Sometimes the solid particles in a liquid, are very small and pass through a filter paper., For such particles the filtration technique, cannot be used for separation. Such mixtures, 19, , IS MATTER AROUND US PURE?, , 2019-2020

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are separated by centrifugation. The principle, is that the denser particles are forced to the, bottom and the lighter particles stay at the, top when spun rapidly., , Applications, •, •, , Applications, Used in diagnostic laboratories for, blood and urine tests., Used in dairies and home to separate, butter from cream., Used in washing machines to squeeze, out water from wet clothes., , •, •, •, , 2.3.3 HOW CAN WE SEPARATE A MIXTURE, OF TWO IMMISCIBLE LIQUIDS?, , Activity ______________ 2.6, •, , Let us try to separate kerosene oil, from water using a separating funnel., Pour the mixture of kerosene oil and, water in a separating funnel (Fig. 2.6)., Let it stand undisturbed for sometime, so that separate layers of oil and water, are formed., Open the stopcock of the separating, funnel and pour out the lower layer of, water carefully., Close the stopcock of the separating, funnel as the oil reaches the stop-cock., , •, •, , •, , •, , Fig. 2.6: Separation of immiscible liquids, , To separate mixture of oil and water., In the extraction of iron from its ore,, the lighter slag is removed from the, top by this method to leave the molten, iron at the bottom in the furnace., , The principle is that immiscible liquids, separate out in layers depending on their, densities., , 2.3.4 HOW CAN WE SEPARATE A MIXTURE, OF SALT AND CAMPHOR?, We have learnt in chapter 1 that camphor, changes directly from solid to gaseous state, on heating. So, to separate such mixtures that, contain a sublimable volatile component from, a non-sublimable impurity, the sublimation, process is used (Fig. 2.7). Some examples of, solids which sublime are ammonium chloride,, naphthalene and anthracene., , Fig. 2.7: Separation of camphor and salt by, sublimation, , 20, , SCIENCE, , 2019-2020

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2.3.5 IS THE DYE, COLOUR?, , IN BLACK INK A SINGLE, , Activity ______________ 2.7, •, •, , •, , •, , •, , Take a thin strip of filter paper., Draw a line on it using a pencil,, approximately 3 cm above the lower, edge [Fig. 2.8 (a)]., Put a small drop of ink (water soluble,, that is, from a sketch pen or fountain, pen) at the centre of the line. Let it dry., Lower the filter paper into a jar/glass/, beaker/test tube containing water so, that the drop of ink on the paper is just, above the water level, as shown in Fig., 2.8(b) and leave it undisturbed., Watch carefully, as the water rises up, on the filter paper. Record your, observations., , This process of separation of components, of a mixture is known as chromatography., Kroma in Greek means colour. This technique, was first used for separation of colours, so, this name was given. Chromatography is the, technique used for separation of those solutes, that dissolve in the same solvent., With the advancement in technology,, newer techniques of chromatography have, been developed. You will study about, chromatography in higher classes., , Applications, To, •, •, •, , separate, colours in a dye, pigments from natural colours, drugs from blood., , 2.3.6 HOW CAN WE SEPARATE A MIXTURE, OF TWO MISCIBLE LIQUIDS?, , Activity ______________ 2.8, •, •, •, Fig. 2.8: Separation of dyes in black ink using, chromatography, , •, •, , Now answer, •, •, •, , •, , What do you observe on the filter paper, as the water rises on it?, Do you obtain different colours on the, filter paper strip?, What according to you, can be the, reason for the rise of the coloured spot, on the paper strip?, , The ink that we use has water as the, solvent and the dye is soluble in it. As the, water rises on the filter paper it takes along, with it the dye particles. Usually, a dye is a, mixture of two or more colours. The coloured, component that is more soluble in water, rises, faster and in this way the colours get, separated., , Let us try to separate acetone and water, from their mixture., Take the mixture in a distillation flask., Fit it with a thermometer., Arrange the apparatus as shown in, Fig. 2.9., Heat the mixture slowly keeping a close, watch at the thermometer., The acetone vaporises, condenses in, the condenser and can be collected, from the condenser outlet., Water is left behind in the distillation, flask., , Fig.2.9: Separation of two miscible liquids by, distillation, 21, , IS MATTER AROUND US PURE?, , 2019-2020

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2.3.7 HOW, , Now answer, •, •, , •, •, , What do you observe as you start, heating the mixture?, At what temperature does the, ther mometer, reading, become, constant for some time?, What is the boiling point of acetone?, Why do the two components separate?, , CAN WE OBTAIN DIFFERENT, , GASES FROM AIR ?, , Air is a homogeneous mixture and can be, separated into its components by fractional, distillation. The flow diagram (Fig. 2.11), shows the steps of the process., , This method is called distillation. It is used, for the separation of components of a mixture, containing two miscible liquids that boil, without decomposition and have sufficient, difference in their boiling points., To separate a mixture of two or more, miscible liquids for which the difference in, boiling points is less than 25 K, fractional, distillation process is used, for example, for, the separation of different gases from air,, different factions from petroleum products, etc. The apparatus is similar to that for simple, distillation, except that a fractionating, column is fitted in between the distillation, flask and the condenser., A simple fractionating column is a tube, packed with glass beads. The beads provide, surface for the vapours to cool and condense, repeatedly, as shown in Fig. 2.10., Fig. 2.11: Flow diagram shows the process of, obtaining gases from air, , If we want oxygen gas from air (Fig. 2.12),, we have to separate out all the other gases, present in the air. The air is compressed by, increasing the pressure and is then cooled, by decreasing the temperature to get liquid, air. This liquid air is allowed to warm-up, slowly in a fractional distillation column,, where gases get separated at different heights, depending upon their boiling points., Answer the following:, •, •, Fig. 2.10: Fractional distillation, 22, , Arrange the gases present in air in, increasing order of their boiling points., Which gas forms the liquid first as the, air is cooled?, SCIENCE, , 2019-2020

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Fig. 2.12: Separation of components of air, , 2.3.8 HOW CAN WE OBTAIN PURE COPPER, SULPHATE FROM AN IMPURE SAMPLE?, , Activity ______________ 2.9, •, , •, •, •, , •, , •, •, , Take some (approximately 5 g) impure, sample of copper sulphate in a china, dish., Dissolve it in minimum amount of, water., Filter the impurities out., Evaporate water from the copper, sulphate solution so as to get a, saturated solution., Cover the solution with a filter paper, and leave it undisturbed at room, temperature to cool slowly for a day., You will obtain the crystals of copper, sulphate in the china dish., This process is called crystallisation., , Now answer, •, •, •, , What do you observe in the china dish?, Do the crystals look alike?, How will you separate the crystals from, the liquid in the china dish?, , The crystallisation method is used to, purify solids. For example, the salt we get, from sea water can have many impurities in, , it. To remove these impurities, the process of, crystallisation is used. Crystallisation is a, process that separates a pure solid in the form, of its crystals from a solution. Crystallisation, technique is better than simple evaporation, technique as –, •, some solids decompose or some, like, sugar, may get charred on heating to, dryness., •, some impurities may remain dissolved, in the solution even after filtration. On, evaporation these contaminate the, solid., , Applications, •, , Purification of salt that we get from sea, water., •, Separation of crystals of alum (phitkari), from impure samples., Thus, by choosing one of the above, methods according to the nature of the, components of a mixture, we get a pure, substance. With advancements in technology, many more methods of separation techniques, have been devised., In cities, drinking water is supplied from, water works. A flow diagram of a typical water, works is shown in Fig. 2.13. From this figure, write down the processes involved to get the, supply of drinking water to your home from, the water works and discuss it in your class., 23, , IS MATTER AROUND US PURE?, , 2019-2020

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Fig. 2.13: Water purification system in water works, , Q, , They differ in odour and inflammability. We, know that oil burns in air whereas water, extinguishes fire. It is this chemical property, of oil that makes it different from water., Burning is a chemical change. During this, process one substance reacts with another, to undergo a change in chemical composition., Chemical change brings change in the, chemical properties of matter and we get new, substances. A chemical change is also called, a chemical reaction., During burning of a candle, both physical, and chemical changes take place. Can you, distinguish these?, , uestions, 1. How will you separate a mixture, containing kerosene and petrol, (difference in their boiling points, is more than 25ºC), which are, miscible with each other?, 2. Name the technique to separate, (i) butter from curd,, (ii) salt from sea-water,, (iii) camphor from salt., 3. What type of mixtures are, separated by the technique of, crystallisation?, , 2.4 Physical and Chemical, Changes, In the previous chapter, we have learnt about, a few physical properties of matter. The, properties that can be observed and specified, like colour, hardness, rigidity, fluidity,, density, melting point, boiling point etc. are, the physical properites., The interconversion of states is a physical, change because these changes occur without, a change in composition and no change in, the chemical nature of the substance., Although ice, water and water vapour all look, different and display different physical, properties, they are chemically the same., Both water and cooking oil are liquid but, their chemical characteristics are different., , Q, , 24, , uestions, 1. Classify the following as, chemical or physical changes:, • cutting of trees,, • melting of butter in a pan,, • rusting of almirah,, • boiling of water to form steam,, • passing of electric current,, through water and the water, breaking down into hydrogen, and oxygen gases,, • dissolving common salt in, water,, • making a fruit salad with raw, fruits, and, • burning of paper and wood., 2. T ry segregating the things, around you as pure substances, or mixtures., , SCIENCE, , 2019-2020

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2.5 What are the Types of Pure, Substances?, , • The number of elements known at, , More to know, , On the basis of their chemical composition,, substances can be classified either as, elements or compounds., , 2.5.1 ELEMENTS, Robert Boyle was the first scientist to use the, term element in 1661. Antoine Laurent, Lavoisier (1743-94), a French chemist, was, the first to establish an experimentally useful, definition of an element. He defined an, element as a basic form of matter that cannot, be broken down into simpler substances by, chemical reactions., Elements can be normally divided into, metals, non-metals and metalloids., Metals usually show some or all of the, following properties:, •, They have a lustre (shine)., •, They have silvery-grey or golden-yellow, colour., •, They conduct heat and electricity., •, They are ductile (can be drawn into, wires)., •, They are malleable (can be hammered, into thin sheets)., •, They are sonorous (make a ringing, sound when hit)., Examples of metals are gold, silver, copper,, iron, sodium, potassium etc. Mercury is the, only metal that is liquid at room temperature., Non-metals usually show some or all of the, following properties:, •, They display a variety of colours., •, They are poor conductors of heat and, electricity., •, They are not lustrous, sonorous or, malleable., Examples of non-metals are hydrogen,, oxygen, iodine, carbon (coal, coke),, bromine, chlorine etc. Some elements have, intermediate properties between those of, metals and non-metals, they are, called metalloids; examples are boron,, silicon, germanium etc., , •, •, •, •, , present are more than 100., Ninety-two elements are naturally, occurring and the rest are manmade., Majority of the elements are solid., Eleven elements are in gaseous, state at room temperature., Two elements are liquid at room, temperature–mercury, and, bromine., Elements, gallium and cesium, become liquid at a temperature, slightly above room temperature, (303 K)., , 2.5.2 COMPOUNDS, A compound is a substance composed of two, or more elements, chemically combined with, one another in a fixed proportion., What do we get when two or more elements, are combined?, , Activity _____________ 2.10, •, , •, , •, , •, , •, •, , •, , Divide the class into two groups. Give, 5 g of iron filings and 3 g of sulphur, powder in a china dish to both the, groups., Group I, Mix and crush iron filings and sulphur, powder., Group II, Mix and crush iron filings and sulphur, powder. Heat this mixture strongly till, red hot. Remove from flame and let the, mixture cool., Groups I and II, Check for magnetism in the material, obtained. Bring a magnet near the, material and check if the material is, attracted towards the magnet., Compare the texture and colour of the, material obtained by the groups., Add carbon disulphide to one part of, the material obtained. Stir well and, filter., Add dilute sulphuric acid or dilute, hydrochloric acid to the other part of, 25, , IS MATTER AROUND US PURE?, , 2019-2020

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Table 2.2: Mixtures and Compounds, Mixtures, , Compounds, , 1. Elements or compounds just mix, together to form a mixture and no, new compound is formed., , 1., , Elements react to form new compounds., , 2. A mixture has a variable composition., , 2., , The composition of each new substance, is always fixed., , 3, A mixture shows the properties of the, constituent substances., , 3., , The new substance has totally different, properties., , 4. The constituents can be seperated, fairly easily by physical methods., , 4., , The constituents can be separated only, by chemical or electrochemical, reactions., , the material obtained.(Note: teacher, supervision is necessary for this, activity)., Perform all the above steps with both, the elements (iron and sulphur), separately., , •, , Now answer, •, , Did the material obtained by the two, groups look the same?, , •, , Which group has obtained a material, with magnetic properties?, , •, , Can we separate the components of the, material obtained?, , •, , On adding dilute sulphuric acid or, dilute hydrochloric acid, did both the, groups obtain a gas? Did the gas in, both the cases smell the same or, different?, , The gas obtained by Group I is hydrogen,, it is colourless, odourless and combustible–, it is not advised to do the combustion test for, hydrogen in the class. The gas obtained by, Group II is hydrogen sulphide. It is a colourless, gas with the smell of rotten eggs., , You must have observed that the products, obtained by both the groups show different, properties, though the starting materials were, the same. Group I has carried out the activity, involving a physical change whereas in case, of Group II, a chemical change (a chemical, reaction) has taken place., •, The material obtained by group I is a, mixture of the two substances. The, substances given are the elements– iron, and sulphur., •, The properties of the mixture are the, same as that of its constituents., •, The material obtained by group II is a, compound., •, On heating the two elements strongly we, get a compound, which has totally, different properties compared to the, combining elements., •, The composition of a compound is the, same throughout. We can also observe, that the texture and the colour of the, compound are the same throughout., Thus, we can summarise the physical, and chemical nature of matter in the, following graphical organiser :, , 26, , SCIENCE, , 2019-2020

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What, you have, learnt, •, •, •, , •, •, , •, , •, , •, , A mixture contains more than one substance (element and/, or compound) mixed in any proportion., Mixtures can be separated into pure substances using, appropriate separation techniques., A solution is a homogeneous mixture of two or more substances., The major component of a solution is called the solvent, and, the minor, the solute., The concentration of a solution is the amount of solute present, per unit volume or per unit mass of the solution., Materials that are insoluble in a solvent and have particles, that are visible to naked eyes, form a suspension. A suspension, is a heterogeneous mixture., Colloids are heterogeneous mixtures in which the particle size, is too small to be seen with the naked eye, but is big enough to, scatter light. Colloids are useful in industry and daily life. The, particles are called the dispersed phase and the medium in, which they are distributed is called the dispersion medium., Pure substances can be elements or compounds. An element, is a form of matter that cannot be broken down by chemical, reactions into simpler substances. A compound is a substance, composed of two or more different types of elements, chemically, combined in a fixed proportion., Properties of a compound are different from its constituent, elements, whereas a mixture shows the properties of its, constituting elements or compounds., 27, , IS MATTER AROUND US PURE?, , 2019-2020

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Exercises, 1., , 2., , 3., , Which separation techniques will you apply for the, separation of the following?, (a) Sodium chloride from its solution in water., (b) Ammonium chloride from a mixture containing sodium, chloride and ammonium chloride., (c) Small pieces of metal in the engine oil of a car., (d) Different pigments from an extract of flower petals., (e) Butter from curd., (f) Oil from water., (g) Tea leaves from tea., (h) Iron pins from sand., (i) Wheat grains from husk., (j) Fine mud particles suspended in water., Write the steps you would use for making tea. Use the words, solution, solvent, solute, dissolve, soluble, insoluble, filtrate, and residue., Pragya tested the solubility of three different substances at, different temperatures and collected the data as given below, (results are given in the following table, as grams of substance, dissolved in 100 grams of water to form a saturated solution)., Substance Dissolved, , Temperature in K, 283, 293, 313, 333, Solubility, , 353, , Potassium nitrate, , 21, , 32, , 62, , 106, , 167, , Sodium chloride, , 36, , 36, , 36, , 37, , 37, , Potassium chloride, , 35, , 35, , 40, , 46, , 54, , Ammonium chloride, , 24, , 37, , 41, , 55, , 66, , (a), , (b), , (c), (d), , What mass of potassium nitrate would be needed to, produce a saturated solution of potassium nitrate in, 50 grams of water at 313 K?, Pragya makes a saturated solution of potassium chloride, in water at 353 K and leaves the solution to cool at room, temperature. What would she observe as the solution, cools? Explain., Find the solubility of each salt at 293 K. Which salt has, the highest solubility at this temperature?, What is the effect of change of temperature on the, solubility of a salt?, , 28, , SCIENCE, , 2019-2020

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4., , 5., , 6., 7., , 8., , 9., , 10., , Explain the following giving examples., (a) saturated solution, (b) pure substance, (c) colloid, (d) suspension, Classify each of the following as a homogeneous or, heterogeneous mixture., soda water, wood, air, soil, vinegar, filtered tea., How would you confirm that a colourless liquid given to you is, pure water?, Which of the following materials fall in the category of a “pure, substance”?, (a) Ice, (b) Milk, (c) Iron, (d) Hydrochloric acid, (e) Calcium oxide, (f), Mercury, (g) Brick, (h) Wood, (i) Air., Identify the solutions among the following mixtures., (a) Soil, (b) Sea water, (c) Air, (d) Coal, (e) Soda water., Which of the following will show “Tyndall effect”?, (a) Salt solution, (b) Milk, (c) Copper sulphate solution, (d) Starch solution., Classify the following into elements, compounds and, mixtures., (a) Sodium, (b) Soil, (c) Sugar solution, (d) Silver, (e) Calcium carbonate, (f) Tin, (g) Silicon, 29, , IS MATTER AROUND US PURE?, , 2019-2020

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11., , (h) Coal, (i) Air, (j) Soap, (k) Methane, (l) Carbon dioxide, (m) Blood, Which of the following are chemical changes?, (a) Growth of a plant, (b) Rusting of iron, (c) Mixing of iron filings and sand, (d) Cooking of food, (e) Digestion of food, (f) Freezing of water, (g) Burning of a candle., , Group Activity, Take an earthen pot (mutka), some pebbles and sand. Design a, small-scale filtration plant that you could use to clean muddy, water., , 30, , SCIENCE, , 2019-2020

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Chapter 3, ATOMS, Ancient Indian and Greek philosophers have, always wondered about the unknown and, unseen form of matter. The idea of divisibility, of matter was considered long back in India,, around 500 BC. An Indian philosopher, Maharishi Kanad, postulated that if we go on, dividing matter (padarth), we shall get smaller, and smaller particles. Ultimately, a stage will, come when we shall come across the smallest, particles beyond which further division will, not be possible. He named these particles, Parm anu. Another Indian philosopher,, Pakudha Katyayama, elaborated this doctrine, and said that these particles normally exist, in a combined form which gives us various, forms of matter., Around the same era, ancient Greek, philosophers – Democritus and Leucippus, suggested that if we go on dividing matter, a, stage will come when particles obtained, cannot be divided further. Democritus called, these indivisible particles atoms (meaning, indivisible). All this was based on, philosophical considerations and not much, experimental work to validate these ideas, could be done till the eighteenth century., By the end of the eighteenth century,, scientists recognised the difference between, elements and compounds and naturally, became interested in finding out how and why, elements combine and what happens when, they combine., Antoine L. Lavoisier laid the foundation, of chemical sciences by establishing two, important laws of chemical combination., , AND, , MOLECULES, , much experimentations by Lavoisier and, Joseph L. Proust., , 3.1.1 LAW OF CONSERVATION OF MASS, Is there a change in mass when a chemical, change (chemical reaction) takes place?, , Activity ______________ 3.1, •, , Take one of the following sets, X and Y, of chemicals–, , X, , •, , •, , •, , Y, , (i) copper sulphate sodium carbonate, 1.25 g, 1.43 g, (ii) barium chloride sodium sulphate, 1.22 g, 1.53 g, (iii) lead nitrate, sodium chloride, 2.07 g, 1.17 g, Prepare separately a 5% solution of, any one pair of substances listed, under X and Y each in 10 mL in water., Take a little amount of solution of Y in, a conical flask and some solution of, X in an ignition tube., Hang the ignition tube in the flask, carefully; see that the solutions do not, get mixed. Put a cork on the flask, (see Fig. 3.1)., , 3.1 Laws of Chemical Combination, The following two laws of chemical, combination were established after, , Fig. 3.1: Ignition tube containing solution of X, dipped, in a conical flask containing solution of Y., , 2019-2020

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conservation of mass and the law of definite, proportions., , Weigh the flask with its contents, carefully., Now tilt and swirl the flask, so that the, solutions X and Y get mixed., Weigh again., What happens in the reaction flask?, Do you think that a chemical reaction, has taken place?, Why should we put a cork on the mouth, of the flask?, Does the mass of the flask and its, contents change?, , •, •, •, •, •, •, •, , Law of conservation of mass states that, mass can neither be created nor destroyed in, a chemical reaction., , 3.1.2 LAW OF CONSTANT PROPORTIONS, Lavoisier, along with other scientists, noted, that many compounds were composed of two, or more elements and each such compound, had the same elements in the same, proportions, irrespective of where the, compound came from or who prepared it., In a compound such as water, the ratio of, the mass of hydrogen to the mass of oxygen is, always 1:8, whatever the source of water. Thus,, if 9 g of water is decomposed, 1 g of hydrogen, and 8 g of oxygen are always obtained., Similarly in ammonia, nitrogen and hydrogen, are always present in the ratio 14:3 by mass,, whatever the method or the source from which, it is obtained., This led to the law of constant proportions, which is also known as the law of definite, proportions. This law was stated by Proust as, “In a chemical substance the elements are, always present in definite proportions by, mass”., The next problem faced by scientists was, to give appropriate explanations of these laws., British chemist John Dalton provided the, basic theory about the nature of matter., Dalton picked up the idea of divisibility of, matter, which was till then just a philosophy., He took the name ‘atoms’ as given by the, Greeks and said that the smallest particles of, matter are atoms. His theory was based on the, laws of chemical combination. Dalton’s atomic, theory provided an explanation for the law of, , John Dalton was born in, a poor weaver’s family in, 1766 in England. He, began his career as a, teacher at the age of, twelve. Seven years later, he became a school, principal. In 1793, Dalton, left for Manchester to, teach mathematics,, John Dalton, physics and chemistry in, a college. He spent most of his life there, teaching and researching. In 1808, he, presented his atomic theory which was a, turning point in the study of matter., According to Dalton’s atomic theory, all, matter, whether an element, a compound or, a mixture is composed of small particles called, atoms. The postulates of this theory may be, stated as follows:, (i) All matter is made of very tiny particles, called atoms, which participate in, chemical reactions., (ii) Atoms are indivisible particles, which, cannot be created or destroyed in a, chemical reaction., (iii) Atoms of a given element are identical, in mass and chemical properties., (iv) Atoms of different elements have, dif ferent masses and chemical, properties., (v) Atoms combine in the ratio of small, whole numbers to form compounds., (vi) The relative number and kinds of, atoms are constant in a given, compound., You will study in the next chapter that all, atoms are made up of still smaller particles., , Q, , 32, , uestions, 1. In a reaction, 5.3 g of sodium, carbonate reacted with 6 g of, acetic acid. The products were, 2.2 g of carbon dioxide, 0.9 g, water and 8.2 g of sodium, acetate. Show that these, SCIENCE, , 2019-2020

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observations are in agreement, with the law of conservation of, mass., sodium carbonate + acetic acid, → sodium acetate + carbon, dioxide + water, 2. Hydrogen and oxygen combine in, the ratio of 1:8 by mass to form, water. What mass of oxygen gas, would be required to react, completely with 3 g of hydrogen, gas?, 3. Which postulate of Dalton’s, atomic theory is the result of the, law of conservation of mass?, 4. Which postulate of Dalton’s, atomic theory can explain the law, of definite proportions?, , We might think that if atoms are so, insignificant in size, why should we care about, them? This is because our entire world is, made up of atoms. We may not be able to see, them, but they are there, and constantly, affecting whatever we do. Through modern, techniques, we can now produce magnified, images of surfaces of elements showing atoms., , 3.2 What is an Atom?, Have you ever observed a mason building, walls, from these walls a room and then a, collection of rooms to form a building? What, is the building block of the huge building?, What about the building block of an ant-hill?, It is a small grain of sand. Similarly, the, building blocks of all matter are atoms., , Fig. 3.2: An image of the surface of silicon, , 3.2.1 W HAT, , SYMBOLS OF ATOMS OF DIFFERENT, ELEMENTS ?, , How big are atoms?, Atoms are very small, they are smaller than, anything that we can imagine or compare, with. More than millions of atoms when, stacked would make a layer barely as thick, as this sheet of paper., Atomic radius is measured in nanometres., 1/10 9 m = 1 nm, 1 m = 109 nm, , A R E THE MODERN DAY, , Dalton was the first scientist to use the, symbols for elements in a very specific sense., When he used a symbol for an element he, also meant a definite quantity of that element,, that is, one atom of that element. Berzilius, suggested that the symbols of elements be, made from one or two letters of the name of, the element., , Relative Sizes, Radii (in m), 10–10, , Example, Atom of hydrogen, , 10–9, , Molecule of water, , 10–8, , Molecule of haemoglobin, , –4, , 10, , Grain of sand, , 10–3, , Ant, , –1, , 10, , ATOMS, , Apple, AND, , Fig. 3.3: Symbols for some elements as proposed by, Dalton, 33, , MOLECULES, , 2019-2020

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In the beginning, the names of elements, were derived from the name of the place where, they were found for the first time. For example,, the name copper was taken from Cyprus. Some, names were taken from specific colours. For, example, gold was taken from the English word, meaning yellow. Now-a-days, IUPAC, (International Union of Pure and Applied, Chemistry) is an international scientific, organisation which approves names of, elements, symbols and units. Many of the, symbols are the first one or two letters of the, element’s name in English. The first letter of a, symbol is always written as a capital letter, (uppercase) and the second letter as a small, letter (lowercase)., For example, (i) hydrogen, H, (ii) aluminium, Al and not AL, (iii) cobalt, Co and not CO., Symbols of some elements are formed from, the first letter of the name and a letter,, appearing later in the name. Examples are: (i), chlorine, Cl, (ii) zinc, Zn etc., Other symbols have been taken from the, names of elements in Latin, German or Greek., For example, the symbol of iron is Fe from its, Latin name ferrum, sodium is Na from natrium,, potassium is K from kalium. Therefore, each, element has a name and a unique chemical, symbol., , passage of time and repeated usage you will, automatically be able to reproduce the, symbols)., , 3.2.2 ATOMIC MASS, The most remarkable concept that Dalton’s, atomic theory proposed was that of the atomic, mass. According to him, each element had a, characteristic atomic mass. The theory could, explain the law of constant proportions so well, that scientists were prompted to measure the, atomic mass of an atom. Since determining the, mass of an individual atom was a relatively, difficult task, relative atomic masses were, determined using the laws of chemical, combinations and the compounds formed., Let us take the example of a compound,, carbon monoxide (CO) formed by carbon and, oxygen. It was observed experimentally that 3, g of carbon combines with 4 g of oxygen to, form CO. In other words, carbon combines, with 4/3 times its mass of oxygen. Suppose, we define the atomic mass unit (earlier, abbreviated as ‘amu’, but according to the, latest IUPAC recommendations, it is now, written as ‘u’ – unified mass) as equal to the, mass of one carbon atom, then we would, , Table 3.1: Symbols for some elements, Element, , Symbol, , Element, , Symbol, , Element, , Symbol, , Aluminium, Argon, Barium, Boron, Bromine, Calcium, Carbon, Chlorine, , Al, Ar, Ba, B, Br, Ca, C, Cl, , Copper, Fluorine, Gold, Hydrogen, Iodine, Iron, Lead, Magnesium, , Cu, F, Au, H, I, Fe, Pb, Mg, , Nitrogen, Oxygen, Potassium, Silicon, Silver, Sodium, Sulphur, Uranium, , N, O, K, Si, Ag, Na, S, U, , Cobalt, , Co, , Neon, , Ne, , Zinc, , Zn, , (The above table is given for you to refer to, whenever you study about elements. Do not, bother to memorise all in one go. With the, , assign carbon an atomic mass of 1.0 u and, oxygen an atomic mass of 1.33 u. However, it, is more convenient to have these numbers as, , 34, , SCIENCE, , 2019-2020

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whole numbers or as near to a whole numbers, as possible. While searching for various, atomic mass units, scientists initially took 1/, 16 of the mass of an atom of naturally, occurring oxygen as the unit. This was, considered relevant due to two reasons:, •, oxygen reacted with a large number of, elements and formed compounds., •, this atomic mass unit gave masses of, most of the elements as whole numbers., However, in 1961 for a universally, accepted atomic mass unit, carbon-12 isotope, was chosen as the standard reference for, measuring atomic masses. One atomic mass, unit is a mass unit equal to exactly one-twelfth, (1/12th) the mass of one atom of carbon-12., The relative atomic masses of all elements, have been found with respect to an atom of, carbon-12., Imagine a fruit seller selling fruits without, any standard weight with him. He takes a, watermelon and says, “this has a mass equal, to 12 units” (12 watermelon units or 12 fruit, mass units). He makes twelve equal pieces of, the watermelon and finds the mass of each fruit, he is selling, relative to the mass of one piece, of the watermelon. Now he sells his fruits by, relative fruit mass unit (fmu), as in Fig. 3.4., , mass of the atom, as compared to 1/12th the, mass of one carbon-12 atom., , Table 3.2: Atomic masses of, a few elements, Element, , Atomic Mass (u), , Hydrogen, , 1, , Carbon, , 12, , Nitrogen, , 14, , Oxygen, , 16, , Sodium, , 23, , Magnesium, , 24, , Sulphur, , 32, , Chlorine, , 35.5, , Calcium, , 40, , 3.2.3 HOW DO ATOMS EXIST?, Atoms of most elements are not able to exist, independently. Atoms form molecules and, ions. These molecules or ions aggregate in, large numbers to form the matter that we can, see, feel or touch., , Q, , uestions, 1. Define the atomic mass unit., 2. Why is it not possible to see an, atom with naked eyes?, , 3.3 What is a Molecule?, , Fig. 3.4 : (a) Watermelon, (b) 12 pieces, (c) 1/12 of, watermelon, (d) how the fruit seller can, weigh the fruits using pieces of watermelon, , Similarly, the relative atomic mass of the, atom of an element is defined as the average, ATOMS, , AND, , A molecule is in general a group of two or, more atoms that are chemically bonded, together, that is, tightly held together by, attractive forces. A molecule can be defined as, the smallest particle of an element or a, compound that is capable of an independent, existence and shows all the properties of that, substance. Atoms of the same element or of, different elements can join together to form, molecules., 35, , MOLECULES, , 2019-2020

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3.3.1 MOLECULES OF ELEMENTS, The molecules of an element are constituted, by the same type of atoms. Molecules of many, elements, such as argon (Ar), helium (He) etc., are made up of only one atom of that element., But this is not the case with most of the nonmetals. For example, a molecule of oxygen, consists of two atoms of oxygen and hence it, is known as a diatomic molecule, O2. If 3, atoms of oxygen unite into a molecule, instead, of the usual 2, we get ozone, O3. The number, of atoms constituting a molecule is known as, its atomicity., Metals and some other elements, such as, carbon, do not have a simple structure but, consist of a very large and indefinite number, of atoms bonded together., Let us look at the atomicity of some, non-metals., , Non-Metal, , Name, , Compound, , Combining, Elements, , Ratio, by, Mass, , Water (H2O), , Hydrogen, Oxygen, , 1:8, , Ammonia (NH3) Nitrogen, Hydrogen 14:3, Carbon, dioxide (CO2), , Carbon, Oxygen, , 3:8, , Activity ______________ 3.2, •, , •, , Refer to Table 3.4 for ratio by mass of, atoms present in molecules and Table, 3.2 for atomic masses of elements. Find, the ratio by number of the atoms of, elements in the molecules of, compounds given in Table 3.4., The ratio by number of atoms for a, water molecule can be found as follows:, , Element Ratio Atomic Mass Simplest, by, mass ratio/, ratio, mass, (u), atomic, mass, , Table 3.3 : Atomicity of some, elements, Type of, Element, , Table 3.4 : Molecules of some, compounds, , Atomicity, , 1, , Argon, , Monoatomic, , Helium, , Monoatomic, , Oxygen, , Diatomic, , Hydrogen, , Diatomic, , Nitrogen, , Diatomic, , Chlorine, , Diatomic, , Phosphorus, , Tetra-atomic, , Sulphur, , Poly-atomic, , H, , 1, , 1, , O, , 8, , 16, , 1, , =1, , 8, , •, , 16, , 2, , 1, , =, , 2, , 1, , Thus, the ratio by number of atoms for, water is H:O = 2:1., , 3.3.3 WHAT IS AN ION?, , 3.3.2 MOLECULES OF COMPOUNDS, Atoms of different elements join together, in definite proportions to form molecules, of compounds. Few examples are given in, Table 3.4., , Compounds composed of metals and nonmetals contain charged species. The charged, species are known as ions. Ions may consist, of a single charged atom or a group of atoms, that have a net charge on them. An ion can be, negatively or positively charged. A negatively, charged ion is called an ‘anion’ and the, positively charged ion, a ‘cation’. Take, for, example, sodium chloride (NaCl). Its, constituent particles are positively charged, sodium ions (Na+) and negatively charged, , 36, , SCIENCE, , 2019-2020

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chloride ions (Cl–). A group of atoms carrying, a charge is known as a polyatomic ion (Table, 3.6). We shall learn more about the formation, of ions in Chapter 4., , Table 3.5: Some ionic compounds, Ionic, Compound, , Constituting, Elements, , Ratio, by, Mass, , Calcium oxide, , Calcium and, oxygen, , 5:2, , Magnesium, sulphide, , Magnesium, and sulphur, , 3:4, , Sodium, chloride, , Sodium, and chlorine, , 23:35.5, , 3.4 Writing Chemical Formulae, The chemical formula of a compound is a, symbolic representation of its composition. The, chemical formulae of different compounds can, be written easily. For this exercise, we need to, , learn the symbols and combining capacity of, the elements., The combining power (or capacity) of an, element is known as its valency. Valency can, be used to find out how the atoms of an, element will combine with the atom(s) of, another element to form a chemical compound., The valency of the atom of an element can be, thought of as hands or arms of that atom., Human beings have two arms and an, octopus has eight. If one octopus has to catch, hold of a few people in such a manner that all, the eight arms of the octopus and both arms, of all the humans are locked, how many, humans do you think the octopus can hold?, Represent the octopus with O and humans, with H. Can you write a formula for this, combination? Do you get OH4 as the formula?, The subscript 4 indicates the number of, humans held by the octopus., The valencies of some common ions are, given in Table 3.6. We will learn more about, valency in the next chapter., , Table 3.6: Names and symbols of some ions, Valency, , Name of, ion, , 1., , Sodium, Potassium, Silver, Copper (I)*, , Symbol, , Na+, K+, Ag+, Cu+, , Nonmetallic, element, , Symbol, , Polyatomic, ions, , Symbol, , Hydrogen, Hydride, Chloride, Bromide, Iodide, , H+, HClBrI–, , Ammonium, Hydroxide, Nitrate, Hydrogen, carbonate, , NH4+, OH–, NO3–, HCO3–, , 2., , Magnesium, Calcium, Zinc, Iron (II)*, Copper (II)*, , Mg2+, Ca2+, Zn2+, Fe2+, Cu2+, , Oxide, Sulphide, , O2S2-, , Carbonate, Sulphite, Sulphate, , CO32–, SO32–, SO42–, , 3., , Aluminium, Iron (III)*, , Al3+, Fe3+, , Nitride, , N3-, , Phosphate, , PO43–, , * Some elements show more than one valency. A Roman numeral shows their valency in a bracket., ATOMS, , AND, , 37, , MOLECULES, , 2019-2020

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The rules that you have to follow while writing, a chemical formula are as follows:, •, the valencies or charges on the ion must, balance., •, when a compound consists of a metal and, a non-metal, the name or symbol of the, metal is written first. For example: calcium, oxide (CaO), sodium chloride (NaCl), iron, sulphide (FeS), copper oxide (CuO) etc.,, where oxygen, chlorine, sulphur are nonmetals and are written on the right,, whereas calcium, sodium, iron and, copper are metals, and are written on the, left., •, in compounds formed with polyatomic ions,, the number of ions present in the, compound is indicated by enclosing the, formula of ion in a bracket and writing the, number of ions outside the bracket. For, example, Mg (OH)2. In case the number of, polyatomic ion is one, the bracket is not, required. For example, NaOH., , 3.4.1 FORMULAE OF SIMPLE COMPOUNDS, The simplest compounds, which are made up, of two different elements are called binary, compounds. Valencies of some ions are given, in Table 3.6. You can use these to write, formulae for compounds., While writing the chemical formulae for, compounds, we write the constituent elements, and their valencies as shown below. Then we, must crossover the valencies of the combining, atoms., , 3. Formula of carbon tetrachloride, , For magnesium chloride, we write the, symbol of cation (Mg2+) first followed by the, symbol of anion (Cl-). Then their charges are, criss-crossed to get the formula., 4., , Formula of magnesium chloride, , Formula : MgCl2, Thus, in magnesium chloride, there are, two chloride ions (Cl-) for each magnesium, ion (Mg2+). The positive and negative charges, must balance each other and the overall, structure must be neutral. Note that in the, formula, the charges on the ions are not, indicated., Some more examples, (a), Formula for aluminium oxide:, , Examples, 1. Formula of hydrogen chloride, Formula : Al2O3, (b), , Formula for calcium oxide:, , Formula of the compound would be HCl., 2. Formula of hydrogen sulphide, , Here, the valencies of the two elements, are the same. You may arrive at the formula, Ca2O2. But we simplify the formula as CaO., 38, , SCIENCE, , 2019-2020

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following formulae:, (i) Al2(SO4)3, (ii) CaCl2, (iii) K2SO4, (iv) KNO3, (v) CaCO3., 3. What is meant by the ter m, chemical formula?, 4. How many atoms are present in a, (i) H2S molecule and, (ii) PO43– ion?, , (c) Formula of sodium nitrate:, , Formula : NaNO3, (d) Formula of calcium hydroxide:, , Formula : Ca(OH)2, Note that the for mula of calcium, hydroxide is Ca(OH)2 and not CaOH2. We use, brackets when we have two or more of the, same ions in the formula. Here, the bracket, around OH with a subscript 2 indicates that, there are two hydroxyl (OH) groups joined to, one calcium atom. In other words, there are, two atoms each of oxygen and hydrogen in, calcium hydroxide., , 3.5 Molecular Mass and Mole, Concept, 3.5.1 MOLECULAR MASS, In section 3.2.2 we discussed the concept of, atomic mass. This concept can be extended, to calculate molecular masses. The molecular, mass of a substance is the sum of the atomic, masses of all the atoms in a molecule of the, substance. It is therefore the relative mass of, a molecule expressed in atomic mass units (u)., , (e) Formula of sodium carbonate:, Example 3.1 (a) Calculate the relative, molecular mass of water (H 2 O)., (b) Calculate the molecular mass of, HNO3., , Formula : Na2CO3, , Solution:, , In the above example, brackets are not needed, if there is only one ion present., , (a) Atomic mass of hydrogen = 1u,, oxygen = 16 u, So the molecular mass of water, which, contains two atoms of hydrogen and one, atom of oxygen is, = 2 × 1+ 1×16, = 18 u, (b) The molecular mass of HNO 3 = the, atomic mass of H + the atomic mass of, N+ 3 × the atomic mass of O, = 1 + 14 + 48 = 63 u, , (f) Formula of ammonium sulphate:, , Formula : (NH4)2SO4, , Q, ATOMS, , uestions, 1. Write down the formulae of, (i) sodium oxide, (ii) aluminium chloride, (iii) sodium suphide, (iv) magnesium hydroxide, 2. Write down the names of, compounds represented by the, AND, , 3.5.2 FORMULA UNIT, , MASS, , The formula unit mass of a substance is a sum, of the atomic masses of all atoms in a formula, unit of a compound. Formula unit mass is, calculated in the same manner as we calculate, the molecular mass. The only difference is that, 39, , MOLECULES, , 2019-2020

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we use the word formula unit for those, substances whose constituent particles are, ions. For example, sodium chloride as, discussed above, has a formula unit NaCl. Its, formula unit mass can be calculated as–, 1 × 23 + 1 × 35.5 = 58.5 u, Example 3.2 Calculate the formula unit, mass of CaCl2., Solution:, Atomic mass of Ca, + (2 × atomic mass of Cl), = 40 + 2 × 35.5 = 40 + 71 = 111 u, , Q, , uestions, 1. Calculate the molecular masses, of H2, O2, Cl2, CO2, CH4, C2H6,, C2H4, NH3, CH3OH., 2. Calculate the formula unit, masses of ZnO, Na2O, K2CO3,, given atomic masses of Zn = 65 u,, Na = 23 u, K = 39 u, C = 12 u,, and O = 16 u., , 3.5.3 MOLE CONCEPT, Take an example of the reaction of hydrogen, and oxygen to form water:, 2H2+ O2 → 2H2O., The above reaction indicates that, (i) two molecules of hydrogen combine, with one molecule of oxygen to form, two molecules of water, or, (ii) 4 u of hydrogen molecules combine, with 32 u of oxygen molecules to form, 36 u of water molecules., We can infer from the above equation that, the quantity of a substance can be, characterised by its mass or the number of, molecules. But, a chemical reaction equation, indicates directly the number of atoms or, molecules taking part in the reaction., Therefore, it is more convenient to refer to the, quantity of a substance in terms of the, number of its molecules or atoms, rather than, their masses. So, a new unit “mole” was, introduced. One mole of any species (atoms,, , Fig. 3.5: Relationship between mole, Avogadro number and mass, , 40, , SCIENCE, , 2019-2020

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molecules, ions or particles) is that quantity, in number having a mass equal to its atomic, or molecular mass in grams., The number of particles (atoms, molecules, or ions) present in 1 mole of any substance is, fixed, with a value of 6.022 × 1023. This is an, experimentally obtained value. This number, is called the Avogadro Constant or Avogadro, Number (represented by N0), named in honour, of the Italian scientist, Amedeo Avogadro., 1 mole (of anything) = 6.022 × 1023 in number,, as, 1 dozen = 12 nos., 1 gross = 144 nos., Besides being related to a number, a mole, has one more advantage over a dozen or a, gross. This advantage is that mass of 1 mole, of a particular substance is also fixed., The mass of 1 mole of a substance is equal, to its relative atomic or molecular mass in, grams. The atomic mass of an element gives, us the mass of one atom of that element in, atomic mass units (u). To get the mass of, 1, mole of atom of that element, that is, molar, mass, we have to take the same numerical, value but change the units from ‘u’ to ‘g’. Molar, mass of atoms is also known as gram atomic, mass. For example, atomic mass of, hydrogen=1u. So, gram atomic mass of, hydrogen = 1 g., 1 u hydrogen has only 1 atom of hydrogen, 1 g hydrogen has 1 mole atoms, that is,, 6.022 × 1023 atoms of hydrogen., Similarly,, 16 u oxygen has only 1 atom of oxygen,, 16 g oxygen has 1 mole atoms, that is,, 6.022 × 1023 atoms of oxygen., To find the gram molecular mass or molar, mass of a molecule, we keep the numerical, value the same as the molecular mass, but, simply change units as above from u to g. For, example, as we have already calculated,, molecular mass of water (H2O) is 18 u. From, here we understand that, 18 u water has only 1 molecule of water,, 18 g water has 1 mole molecules of water, that, is, 6.022 × 1023 molecules of water., Chemists need the number of atoms and, molecules while carrying out reactions, and, , ATOMS, , AND, , for this they need to relate the mass in grams, to the number. It is done as follows:, 1 mole = 6.022 × 1023 number, = Relative mass in grams., Thus, a mole is the chemist’s counting unit., The word “mole” was introduced around, 1896 by Wilhelm Ostwald who derived the, term from the Latin word moles meaning a, ‘heap’ or ‘pile’. A substance may be considered, as a heap of atoms or molecules. The unit, mole was accepted in 1967 to provide a simple, way of reporting a large number– the massive, heap of atoms and molecules in a sample., Example 3.3, 1. Calculate the number of moles for the, following:, (i) 52 g of He (finding mole from, mass), (ii) 12.044 × 1023 number of He atoms, (finding mole from number of, particles)., Solutions:, No. of moles, Given mass, Molar mass, Given number of particles, Avogadro number of particles, , =, =, =, =, =, , (i), , = 4u, = 4g, , Atomic mass of He, Molar mass of He, , n, m, M, N, N0, , Thus, the number of moles, given mass, , = molar mass, ⇒n=, , (ii), , m 52, =, = 13, M, 4, , we know,, 1 mole = 6.022 × 1023, The number of moles, =, , given number of particles, Avogadro number, , ⇒n=, , N, No, , =, , 12.044 × 1023, 6.022 × 1023, , =2, , 41, , MOLECULES, , 2019-2020

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Example 3.4 Calculate the mass of the, following:, (i) 0.5 mole of N2 gas (mass from mole, of molecule), (ii) 0.5 mole of N atoms (mass from, mole of atom), (iii) 3.011 × 1023 number of N atoms, (mass from number), (iv) 6.022 × 10 23 number of N 2, molecules (mass from number), , (iii) 0.1 mole of carbon atoms (number, from given moles), Solutions:, (i) The number of atoms, =, , ⇒N=, , Solutions:, (i), , (ii), , given mass, × Avogadro number, molar mass, m, × N0, M, , 46, , × 6.022 × 1023, 23, ⇒ N = 12.044 × 1023, ⇒N=, , mass = molar mass × number of, moles, ⇒ m = M × n = 28 × 0.5 = 14 g, , (ii) The number of molecules, , mass = molar mass × number of, moles, ⇒ m = M × n = 14 × 0.5 = 7 g, , =, , given mass, × Avogadro number, molar mass, , m, × N0, M, atomic mass of oxygen =16 u, , ⇒N=, , (iii) The number of moles, n, =, , given number of particles, , =, , N, , =, , Avogadro number, , ∴ molar mass of O2 molecules, = 16 × 2 = 32g, , N0, , 3.011 × 10 23, , 8, × 6.022 × 1023, 32, ⇒ N = 1.5055 × 1023, , 6.022 × 10 23, , ⇒ m = M × n = 14 ×, , ⇒N=, 3.011 × 1023, 6.022 × 1023, , ; 1.51 × 1023, , = 14 × 0.5 = 7 g, , (iv), , n=, , (iii) The number of particles (atom) =, number of moles of particles ×, Avogadro number, N = n × N0 = 0.1 x 6.022 × 1023, = 6.022 × 1022, , N, N0, , ⇒m= M ×, , N, N0, , = 28 ×, , 6.022 × 1023, 6.022 × 1023, , = 28 × 1 = 28 g, , Example 3.5 Calculate the number of, particles in each of the, following:, (i) 46 g of Na atoms (number from, mass), (ii) 8 g O 2 molecules (number of, molecules from mass), , Q, , 42, , uestions, 1. If one mole of carbon atoms, weighs 12 grams, what is the, mass (in grams) of 1 atom of, carbon?, 2. Which has more number of, atoms, 100 grams of sodium or, 100 grams of iron (given, atomic, mass of Na = 23 u, Fe = 56 u)?, , SCIENCE, , 2019-2020

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What, you have, learnt, •, , •, , •, , •, , •, •, •, •, •, , •, •, , •, , During a chemical reaction, the sum of the masses of the, reactants and products remains unchanged. This is known, as the Law of Conservation of Mass., In a pure chemical compound, elements are always present in, a definite proportion by mass. This is known as the Law of, Definite Proportions., An atom is the smallest particle of the element that cannot, usually exist independently and retain all its chemical, properties., A molecule is the smallest particle of an element or a compound, capable of independent existence under ordinary conditions., It shows all the properties of the substance., A chemical formula of a compound shows its constituent, elements and the number of atoms of each combining element., Clusters of atoms that act as an ion are called polyatomic ions., They carry a fixed charge on them., The chemical formula of a molecular compound is determined, by the valency of each element., In ionic compounds, the charge on each ion is used to, determine the chemical formula of the compound., Scientists use the relative atomic mass scale to compare the, masses of different atoms of elements. Atoms of carbon-12, isotopes are assigned a relative atomic mass of 12 and the, relative masses of all other atoms are obtained by comparison, with the mass of a carbon-12 atom., The Avogadro constant 6.022 × 1023 is defined as the number, of atoms in exactly 12 g of carbon-12., The mole is the amount of substance that contains the same, number of particles (atoms/ ions/ molecules/ formula units, etc.) as there are atoms in exactly 12 g of carbon-12., Mass of 1 mole of a substance is called its molar mass., , Exercises, 1. A 0.24 g sample of compound of oxygen and boron was found, by analysis to contain 0.096 g of boron and 0.144 g of oxygen., Calculate the percentage composition of the compound by, weight., 2. When 3.0 g of carbon is burnt in 8.00 g oxygen, 11.00 g of, carbon dioxide is produced. What mass of carbon dioxide will, ATOMS, , AND, , 43, , MOLECULES, , 2019-2020

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3., 4., , 5., , 6., , 7., , 8., , 9., , 10., 11., , be formed when 3.00 g of carbon is burnt in 50.00 g of, oxygen? Which law of chemical combination will govern your, answer?, What are polyatomic ions? Give examples., Write the chemical formulae of the following., (a) Magnesium chloride, (b) Calcium oxide, (c) Copper nitrate, (d) Aluminium chloride, (e) Calcium carbonate., Give the names of the elements present in the following, compounds., (a) Quick lime, (b) Hydrogen bromide, (c) Baking powder, (d) Potassium sulphate., Calculate the molar mass of the following substances., (a) Ethyne, C2H2, (b) Sulphur molecule, S8, (c) Phosphorus molecule, P4 (Atomic mass of phosphorus, = 31), (d) Hydrochloric acid, HCl, (e) Nitric acid, HNO3, What is the mass of—, (a) 1 mole of nitrogen atoms?, (b) 4 moles of aluminium atoms (Atomic mass of aluminium, = 27)?, (c) 10 moles of sodium sulphite (Na2SO3)?, Convert into mole., (a) 12 g of oxygen gas, (b) 20 g of water, (c) 22 g of carbon dioxide., What is the mass of:, (a) 0.2 mole of oxygen atoms?, (b) 0.5 mole of water molecules?, Calculate the number of molecules of sulphur (S8) present in, 16 g of solid sulphur., Calculate the number of aluminium ions present in 0.051 g, of aluminium oxide., (Hint: The mass of an ion is the same as that of an atom of, the same element. Atomic mass of Al = 27 u), , 44, , SCIENCE, , 2019-2020

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Group Activity, Play a game for writing formulae., Example1 : Make placards with symbols and valencies of the, elements separately. Each student should hold two, placards, one with the symbol in the right hand and, the other with the valency in the left hand. Keeping, the symbols in place, students should criss-cross their, valencies to form the formula of a compound., Example 2 : A low cost model for writing formulae: Take empty, blister packs of medicines. Cut them in groups,, according to the valency of the element, as shown in, the figure. Now, you can make formulae by fixing, one type of ion into other., For example:, Na+, , SO 4 2-, , P0 43-, , Formula for sodium sulphate:, 2 sodium ions can be fixed on one sulphate ion., Hence, the formula will be: Na2SO4, Do it yourself :, Now, write the formula of sodium phosphate., , ATOMS, , AND, , 45, , MOLECULES, , 2019-2020

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Chapter 4, STRUCTURE OF THE ATOM, In Chapter 3, we have learnt that atoms and, molecules are the fundamental building, blocks of matter. The existence of different, kinds of matter is due to different atoms, constituting them. Now the questions arise:, (i) What makes the atom of one element, different from the atom of another element?, and (ii) Are atoms really indivisible, as, proposed by Dalton, or are there smaller, constituents inside the atom? We shall find, out the answers to these questions in this, chapter. We will learn about sub-atomic, particles and the various models that have, been proposed to explain how these particles, are arranged within the atom., A major challenge before the scientists at, the end of the 19th century was to reveal the, structure of the atom as well as to explain its, important properties. The elucidation of the, structure of atoms is based on a series of, experiments., One of the first indications that atoms are, not indivisible, comes from studying static, electricity and the condition under which, electricity is conducted by dif ferent, substances., , 4.1 Charged Particles in Matter, For understanding the nature of charged, particles in matter, let us carry out the, following activities:, , Activity ______________ 4.1, A., B., , Comb dry hair. Does the comb then, attract small pieces of paper?, Rub a glass rod with a silk cloth and, bring the rod near an inflated balloon., Observe what happens., , From these activities, can we conclude, that on rubbing two objects together, they, become electrically charged? Where does this, charge come from? This question can be, answered by knowing that an atom is divisible, and consists of charged particles., Many scientists contributed in revealing, the presence of charged particles in an atom., It was known by 1900 that the atom was, indivisible particle but contained at least one, sub-atomic particle – the electron identified by, J.J. Thomson. Even before the electron was, identified, E. Goldstein in 1886 discovered the, presence of new radiations in a gas discharge, and called them canal rays. These rays were, positively charged radiations which ultimately, led to the discovery of another sub-atomic, particle. This sub-atomic particle had a charge,, equal in magnitude but opposite in sign to that, of the electron. Its mass was approximately, 2000 times as that of the electron. It was given, the name of proton. In general, an electron is, represented as ‘e–’ and a proton as ‘p+’. The, mass of a proton is taken as one unit and its, charge as plus one. The mass of an electron is, considered to be negligible and its charge is, minus one., It seemed that an atom was composed of, protons and electrons, mutually balancing, their charges. It also appeared that the protons, were in the interior of the atom, for whereas, electrons could easily be removed off but, not protons. Now the big question was:, what sort of structure did these particles of, the atom form? We will find the answer to, this question below., , 2019-2020

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Q, , uestions, , J.J. Thomson (18561940),, a, British, physicist, was born in, Cheetham Hill, a suburb, of Manchester, on, 18 December 1856. He, was awarded the Nobel, prize in Physics in 1906, for his work on the, discovery of electrons., He directed the Cavendish Laboratory at, Cambridge for 35 years and seven of his, research assistants subsequently won, Nobel prizes., , 1. What are canal rays?, 2. If an atom contains one electron, and one proton, will it carry any, charge or not?, , 4.2 The Structure of an Atom, We have learnt Dalton’s atomic theory in, Chapter 3, which suggested that the atom, was indivisible and indestructible. But the, discovery of two fundamental particles, (electrons and protons) inside the atom, led, to the failure of this aspect of Dalton’s atomic, theory. It was then considered necessary to, know how electrons and protons are arranged, within an atom. For explaining this, many, scientists proposed various atomic models., J.J. Thomson was the first one to propose a, model for the structure of an atom., , 4.2.1 THOMSON’S MODEL OF AN ATOM, Thomson proposed the model of an atom to, be similar to that of a Christmas pudding., The electrons, in a sphere of positive charge,, were like currants (dry fruits) in a spherical, Christmas pudding. We can also think of a, watermelon, the positive charge in the atom, is spread all over like the red edible part of, the watermelon, while the electrons are, studded in the positively charged sphere, like, the seeds in the watermelon (Fig. 4.1)., , Fig.4.1: Thomson’s model of an atom, STRUCTURE, , OF THE, , Thomson proposed that:, (i) An atom consists of a positively, charged sphere and the electrons are, embedded in it., (ii) The negative and positive charges are, equal in magnitude. So, the atom as a, whole is electrically neutral., Although Thomson’s model explained that, atoms are electrically neutral, the results of, experiments carried out by other scientists, could not be explained by this model, as we, will see below., , 4.2.2 RUTHERFORD’S MODEL OF AN ATOM, Ernest Rutherford was interested in knowing, how the electrons are arranged within an, atom. Rutherford designed an experiment for, this. In this experiment, fast moving alpha, (α)-particles were made to fall on a thin, gold foil., •, He selected a gold foil because he wanted, as thin a layer as possible. This gold foil, was about 1000 atoms thick., •, α-particles are doubly-charged helium, ions. Since they have a mass of 4 u, the, fast-moving α-particles have a, considerable amount of energy., •, It was expected that α-particles would be, deflected by the sub-atomic particles in, the gold atoms. Since the α-particles were, much heavier than the protons, he did, not expect to see large deflections., 47, , ATOM, , 2019-2020

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Fig. 4.2: Scattering of α-particles by a gold foil, , But, the α-particle scattering experiment, gave totally unexpected results (Fig. 4.2). The, following observations were made:, (i) Most of the fast moving α-particles, passed straight through the gold foil., (ii) Some of the α-particles were deflected, by the foil by small angles., (iii) Surprisingly one out of every 12000, particles appeared to rebound., In the words of Rutherford, “This result, was almost as incredible as if you fire a, 15-inch shell at a piece of tissue paper and it, comes back and hits you”., E. Rutherford (1871-1937), was born at Spring Grove, on 30 August 1871. He was, known as the ‘Father’ of, nuclear physics. He is, famous for his work on, radioactivity and the, discovery of the nucleus of an atom with, the gold foil experiment. He got the Nobel, prize in chemistry in 1908., , hear a sound when each stone strikes the wall., If he repeats this ten times, he will hear the, sound ten times. But if a blind-folded child, were to throw stones at a barbed-wire fence,, most of the stones would not hit the fencing, and no sound would be heard. This is because, there are lots of gaps in the fence which allow, the stone to pass through them., Following a similar reasoning, Rutherford, concluded from the α-particle scattering, experiment that–, (i) Most of the space inside the atom is, empty because most of the α-particles, passed through the gold foil without, getting deflected., (ii) Very few particles were deflected from, their path, indicating that the positive, charge of the atom occupies very little, space., (iii) A very small fraction of α-particles, were deflected by 1800, indicating that, all the positive charge and mass of the, gold atom were concentrated in a very, small volume within the atom., From the data he also calculated that the, radius of the nucleus is about 105 times less, than the radius of the atom., On the basis of his experiment,, Rutherford put forward the nuclear model of, an atom, which had the following features:, (i) There is a positively charged centre in, an atom called the nucleus. Nearly all, the mass of an atom resides in the, nucleus., (ii) The electrons revolve around the, nucleus in circular paths., (iii) The size of the nucleus is very small, as compared to the size of the atom., Drawbacks of Rutherford’s model of, the atom, , Let us think of an activity in an open field, to understand the implications of this, experiment. Let a child stand in front of a, wall with his eyes closed. Let him throw, stones at the wall from a distance. He will, , The revolution of the electron in a circular orbit, is not expected to be stable. Any particle in a, circular orbit would undergo acceleration., During acceleration, charged particles would, radiate energy. Thus, the revolving electron, would lose energy and finally fall into the, nucleus. If this were so, the atom should be, highly unstable and hence matter would not, exist in the form that we know. We know that, atoms are quite stable., , 48, , SCIENCE, , 2019-2020

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4.2.3 BOHR’S MODEL OF ATOM, In order to overcome the objections raised, against Rutherford’s model of the atom,, Neils Bohr put forward the following, postulates about the model of an atom:, (i) Only certain special orbits known as, discrete orbits of electrons, are allowed, inside the atom., (ii) While revolving in discrete orbits the, electrons do not radiate energy., , Neils Bohr (1885-1962), was born in Copenhagen, on 7 October 1885. He was, appointed professor of, physics at Copenhagen, University in 1916. He got, the Nobel prize for his work, on the structure of atom in, 1922. Among Professor, Bohr’s numerous writings, three appearing, as books are:, (i) The Theory of Spectra and Atomic, Constitution, (ii) Atomic Theory and,, (iii) The Description of Nature., These orbits or shells are called energy, levels. Energy levels in an atom are shown in, Fig. 4.3., , Q, , uestions, 1. On the basis of Thomson’s model, of an atom, explain how the atom, is neutral as a whole., 2. On the basis of Rutherford’s, model of an atom, which subatomic particle is present in the, nucleus of an atom?, 3. Draw a sketch of Bohr’s model, of an atom with three shells., 4. What do you think would be the, observation if the α -particle, scattering experiment is carried, out using a foil of a metal other, than gold?, , 4.2.4 NEUTRONS, In 1932, J. Chadwick discovered another subatomic particle which had no charge and a, mass nearly equal to that of a proton. It was, eventually named as neutron. Neutrons are, present in the nucleus of all atoms, except, hydr ogen. In general, a neutron is, represented as ‘n’. The mass of an atom is, therefore given by the sum of the masses of, protons and neutrons present in the nucleus., , Q, , uestions, 1. Name the three sub-atomic, particles of an atom., 2. Helium atom has an atomic mass, of 4 u and two protons in its, nucleus. How many neutrons, does it have?, , 4.3 How are Electrons Distributed, in Different Orbits (Shells)?, , Fig. 4.3: A few energy levels in an atom, , These orbits or shells are represented by, the letters K,L,M,N,… or the numbers,, n=1,2,3,4,…., STRUCTURE, , OF THE, , The distribution of electrons into different, orbits of an atom was suggested by Bohr and, Bury., The following rules are followed for writing, the number of electrons in different energy, levels or shells:, (i) The maximum number of electrons, present in a shell is given by the, 49, , ATOM, , 2019-2020

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formula 2n 2, where ‘n’ is the orbit, number or energy level index, 1,2,3,…., Hence the maximum number of, electrons in different shells are as, follows:, first orbit or K-shell will be = 2 × 12 = 2,, second orbit or L-shell will be = 2 × 22 = 8,, third orbit or M-shell will be = 2 × 32 = 18,, fourth orbit or N-shell will be = 2 × 42, = 32, and so on., (ii) The maximum number of electrons that, can be accommodated in the outermost, orbit is 8., (iii) Electrons are not accommodated in a, given shell, unless the inner shells are, filled. That is, the shells are filled in a, step-wise manner., Atomic structure of the first eighteen, elements is shown schematically in Fig. 4.4., , •, , Q, , The composition of atoms of the first, eighteen elements is given in Table 4.1., , uestions, 1. Write the distribution of electrons, in carbon and sodium atoms., 2. If K and L shells of an atom are, full, then what would be the total, number of electrons in the atom?, , 4.4 Valency, We have learnt how the electrons in an atom, are arranged in different shells/orbits. The, electrons present in the outermost shell of, an atom are known as the valence electrons., From the Bohr-Bury scheme, we also know, that the outermost shell of an atom can, , Fig.4.4: Schematic atomic structure of the first eighteen elements, , Activity ______________ 4.2, Make a static atomic model displaying, electronic configuration of the first, eighteen elements., , •, , accommodate a maximum of 8 electrons. It, was observed that the atoms of elements,, completely filled with 8 electrons in the, outermost shell show little chemical activity., In other words, their combining capacity or, valency is zero. Of these inert elements, the, , 50, , SCIENCE, , 2019-2020

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Table 4.1: Composition of Atoms of the First Eighteen Elements, with Electron Distribution in Various Shells, Name of, Element, , Symbol, , Atomic Number Number Number, Number, of, of, of, Protons Neutrons Electrons, , Distribution of, Electrons, , Valency, , K, , L, , M, , N, , Hydrogen, Helium, , H, He, , 1, 2, , 1, 2, , 2, , 1, 2, , 1, 2, , -, , -, , -, , 1, 0, , Lithium, , Li, , 3, , 3, , 4, , 3, , 2, , 1, , -, , -, , 1, , Beryllium, , Be, , 4, , 4, , 5, , 4, , 2, , 2, , -, , -, , 2, , Boron, , B, , 5, , 5, , 6, , 5, , 2, , 3, , -, , -, , 3, , Carbon, , C, , 6, , 6, , 6, , 6, , 2, , 4, , -, , -, , 4, , Nitrogen, , N, , 7, , 7, , 7, , 7, , 2, , 5, , -, , -, , 3, , Oxygen, , O, , 8, , 8, , 8, , 8, , 2, , 6, , -, , -, , 2, , Fluorine, , F, , 9, , 9, , 10, , 9, , 2, , 7, , -, , -, , 1, , Neon, , Ne, , 10, , 10, , 10, , 10, , 2, , 8, , -, , -, , 0, , Sodium, , Na, , 11, , 11, , 12, , 11, , 2, , 8, , 1, , -, , 1, , Magnesium, , Mg, , 12, , 12, , 12, , 12, , 2, , 8, , 2, , -, , 2, , Aluminium, , Al, , 13, , 13, , 14, , 13, , 2, , 8, , 3, , -, , 3, , Silicon, , Si, , 14, , 14, , 14, , 14, , 2, , 8, , 4, , -, , 4, , Phosphorus, , P, , 15, , 15, , 16, , 15, , 2, , 8, , 5, , -, , 3,5, , Sulphur, , S, , 16, , 16, , 16, , 16, , 2, , 8, , 6, , -, , 2, , Chlorine, , Cl, , 17, , 17, , 18, , 17, , 2, , 8, , 7, , -, , 1, , Argon, , Ar, , 18, , 18, , 22, , 18, , 2, , 8, , 8, , helium atom has two electrons in its outermost, shell and all other elements have atoms with, eight electrons in the outermost shell., The combining capacity of the atoms of, elements, that is, their tendency to react and, form molecules with atoms of the same or, different elements, was thus explained as an, attempt to attain a fully-filled outermost shell., An outermost-shell, which had eight electrons, was said to possess an octet. Atoms would, thus react, so as to achieve an octet in the, outermost shell. This was done by sharing,, gaining or losing electrons. The number of, electrons gained, lost or shared so as to make, the octet of electrons in the outermost shell,, gives us directly the combining capacity of the, STRUCTURE, , OF THE, , 0, , element, that is, the valency discussed in the, previous chapter. For example, hydrogen/, lithium/sodium atoms contain one electron, each in their outermost shell, therefore each, one of them can lose one electron. So, they are, said to have valency of one. Can you tell, what, is valency of magnesium and aluminium? It, is two and three, respectively, because, magnesium has two electrons in its outermost, shell and aluminium has three electrons in, its outermost shell., If the number of electr ons in the, outermost shell of an atom is close to its full, capacity, then valency is determined in a, different way. For example, the fluorine atom, has 7 electrons in the outermost shell, and, its valency could be 7. But it is easier for, 51, , ATOM, , 2019-2020

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fluorine to gain one electron instead of losing, seven electrons. Hence, its valency is determined, by subtracting seven electrons from the octet, and this gives you a valency of one for fluorine., Valency can be calculated in a similar manner, for oxygen. What is the valency of oxygen that, you get from this calculation?, Therefore, an atom of each element has a, definite combining capacity, called its valency., Valency of the first eighteen elements is given, in the last column of Table 4.1., , Q, , 6 neutrons, 6 u + 6 u = 12 u. Similarly, the, mass of aluminium is 27 u (13 protons+14, neutrons). The mass number is defined as the, sum of the total number of protons and, neutrons present in the nucleus of an atom. It, is denoted by ‘A’. In the notation for an atom,, the atomic number, mass number and symbol, of the element are to be written as:, Mass Number, , uestion, , Atomic Number, , 1. How will you find the valency, of chlorine, sulphur and, magnesium?, , For example, nitrogen is written as, , 4.5 Atomic Number and Mass, Number, 4.5.1 ATOMIC NUMBER, We know that protons are present in the, nucleus of an atom. It is the number of, protons of an atom, which determines its, atomic number. It is denoted by ‘Z’. All atoms, of an element have the same atomic number,, Z. In fact, elements are defined by the number, of protons they possess. For hydrogen, Z = 1,, because in hydrogen atom, only one proton, is present in the nucleus. Similarly, for, carbon, Z = 6. Therefore, the atomic number, is defined as the total number of protons, present in the nucleus of an atom., , A, , Symbol of, element, , Q, , X, Z, 14, 7 N, , ., , uestions, 1. If number of electrons in an atom, is 8 and number of protons is also, 8, then (i) what is the atomic, number of the atom? and (ii) what, is the charge on the atom?, 2. With the help of Table 4.1, find, out the mass number of oxygen, and sulphur atom., , 4.6 Isotopes, In nature, a number of atoms of some, elements have been identified, which have the, same atomic number but different mass, numbers. For example, take the case of, hydrogen atom, it has three atomic species,, namely protium ( 11 H), deuterium ( 12 H or D), 3, , and tritium ( 1 H or T). The atomic number of, each one is 1, but the mass number is 1, 2, and 3, respectively. Other such examples are, , 4.5.2 MASS NUMBER, After studying the properties of the subatomic particles of an atom, we can conclude, that mass of an atom is practically due to, protons and neutrons alone. These are, present in the nucleus of an atom. Hence, protons and neutrons are also called, nucleons. Therefore, the mass of an atom, resides in its nucleus. For example, mass of, carbon is 12 u because it has 6 protons and, , (i) carbon,, and, , 37, 17, , 12, 6, , C and, , 14, 6, , C, (ii) chlorine,, , 35, Cl, 17, , Cl, etc., , On the basis of these examples, isotopes, are defined as the atoms of the same element,, having the same atomic number but different, mass numbers. Therefore, we can say that, there are three isotopes of hydrogen atom,, namely protium, deuterium and tritium., , 52, , SCIENCE, , 2019-2020

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Many elements consist of a mixture of, isotopes. Each isotope of an element is a pure, substance. The chemical properties of, isotopes are similar but their physical, properties are different., Chlorine occurs in nature in two isotopic, forms, with masses 35 u and 37 u in the ratio, of 3:1. Obviously, the question arises: what, should we take as the mass of chlorine atom?, Let us find out., The average atomic mass of chlorine atom,, on the basis of above data, will be, 75, 25 , ,  35 × 100 + 37 × 100 , , , Applications, Since the chemical properties of all the, isotopes of an element are the same,, normally we are not concerned about, taking a mixture. But some isotopes have, special properties which find them useful, in various fields. Some of them are :, (i) An isotope of uranium is used as a fuel, in nuclear reactors., (ii) An isotope of cobalt is used in the, treatment of cancer., (iii) An isotope of iodine is used in the, treatment of goitre., , 4.6.1 ISOBARS, , ,  105 37  142, =, +, =, = 35.5 u , ,  4, 4 , 4, , The mass of an atom of any natural element, is taken as the average mass of all the naturally, occuring atoms of that element. If an element, has no isotopes, then the mass of its atom, would be the same as the sum of protons and, neutrons in it. But if an element occurs in, isotopic forms, then we have to know the, percentage of each isotopic form and then the, average mass is calculated., This does not mean that any one atom of, chlorine has a fractional mass of 35.5 u. It, means that if you take a certain amount of, chlorine, it will contain both isotopes of, chlorine and the average mass is 35.5 u., , Let us consider two elements — calcium,, atomic number 20, and argon, atomic, number 18. The number of protons in these, atoms is different, but the mass number of, both these elements is 40. That is, the total, number of nucleons is the same in the atoms, of this pair of elements. Atoms of different, elements with different atomic numbers, which, have the same mass number, are known as, isobars., , Q, , uestions, 1. For the symbol H,D and T, tabulate three sub-atomic, particles found in each of them., 2. Write, the, electronic, configuration of any one pair of, isotopes and isobars., , What, you have, learnt, •, •, , STRUCTURE, , OF THE, , Credit for the discovery of electron and proton goes to J.J., Thomson and E.Goldstein, respectively., J.J. Thomson proposed that electrons are embedded in a, positive sphere., 53, , ATOM, , 2019-2020

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•, , Rutherford’s alpha-particle scattering experiment led to the, discovery of the atomic nucleus., Rutherford’s model of the atom proposed that a very tiny, nucleus is present inside the atom and electrons revolve, around this nucleus. The stability of the atom could not be, explained by this model., Neils Bohr’s model of the atom was more successful. He, proposed that electrons are distributed in different shells with, discrete energy around the nucleus. If the atomic shells are, complete, then the atom will be stable and less reactive., J. Chadwick discovered presence of neutrons in the nucleus of, an atom. So, the three sub-atomic particles of an atom are:, (i) electrons, (ii) protons and (iii) neutrons. Electrons are, negatively charged, protons are positively charged and neutrons, , •, , •, , •, , have no charges. The mass of an electron is about, , 1, times, 2000, , the mass of an hydrogen atom. The mass of a proton and a, neutron is taken as one unit each., Shells of an atom are designated as K,L,M,N,…., Valency is the combining capacity of an atom., The atomic number of an element is the same as the number, of protons in the nucleus of its atom., The mass number of an atom is equal to the number of nucleons, in its nucleus., Isotopes are atoms of the same element, which have different, mass numbers., Isobars are atoms having the same mass number but different, atomic numbers., Elements are defined by the number of protons they possess., , •, •, •, •, •, •, •, , Exercises, 1., 2., 3., 4., 5., , Compare the properties of electrons, protons and neutrons., What are the limitations of J.J. Thomson’s model of the atom?, What are the limitations of Rutherford’s model of the atom?, Describe Bohr’s model of the atom., Compare all the proposed models of an atom given in this, chapter., 6. Summarise the rules for writing of distribution of electrons in, various shells for the first eighteen elements., 7. Define valency by taking examples of silicon and oxygen., , 54, , SCIENCE, , 2019-2020

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8. Explain with examples (i) Atomic number, (ii) Mass number,, (iii) Isotopes and iv) Isobars. Give any two uses of isotopes., 9. Na+ has completely filled K and L shells. Explain., 10. If bromine atom is available in the form of, say, two isotopes, 79, 35 Br, , (49.7%) and, , 81, 35 Br, , (50.3%), calculate the average atomic, , mass of bromine atom., 11. The average atomic mass of a sample of an element X is 16.2 u., What are the percentages of isotopes, , 16, 8, , X and, , 18, 8, , X in the, , sample?, 12. If Z = 3, what would be the valency of the element? Also, name, the element., 13. Composition of the nuclei of two atomic species X and Y are, given as under, X, , Y, , Protons =, 6, 6, Neutrons =, 6, 8, Give the mass numbers of X and Y. What is the relation between, the two species?, 14. For the following statements, write T for True and F for False., (a) J.J. Thomson proposed that the nucleus of an atom, contains only nucleons., (b) A neutron is formed by an electron and a proton, combining together. Therefore, it is neutral., (c), , The mass of an electron is about, , 1, times that of proton., 2000, , (d), , An isotope of iodine is used for making tincture iodine,, which is used as a medicine., Put tick (ü) against correct choice and cross (×) against, wrong choice in questions 15, 16 and 17, 15. Rutherford’s alpha-particle scattering experiment was, responsible for the discovery of, (a) Atomic Nucleus, (b) Electron, (c) Proton, (d) Neutron, 16. Isotopes of an element have, (a) the same physical properties, (b) different chemical properties, (c) different number of neutrons, (d) different atomic numbers., 17. Number of valence electrons in Cl– ion are:, (a) 16, (b) 8, (c) 17, (d) 18, STRUCTURE, , OF THE, , 55, , ATOM, , 2019-2020

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18. Which one of the following is a correct electronic configuration, of sodium?, (a) 2,8, (b) 8,2,1 (c) 2,1,8, (d) 2,8,1., 19. Complete the following table., Atomic Mass, Number Number, , Number Number Number, Name of, of, of, of, the Atomic, Neutrons Protons Electrons, Species, , 9, , -, , 10, , -, , -, , -, , 16, , 32, , -, , -, , -, , Sulphur, , -, , 24, , -, , 12, , -, , -, , -, , 2, , -, , 1, , -, , -, , -, , 1, , 0, , 1, , 0, , -, , 56, , SCIENCE, , 2019-2020

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Chapter 5, THE FUNDAMENTAL UNIT, , Eyepiece, , Coarse adjustment, Body tube, Fine adjustment, Clip, Microscope slide, Condenser, , 5.1 What are Living Organisms, Made Up of?, , •, , Arm, Objective lens, Stage, Swivel, Mirror, Base, , Activity ______________ 5.1, Let us take a small piece from an onion, bulb. With the help of a pair of forceps,, we can peel of f the skin (called, epidermis) from the concave side (inner, layer) of the onion. This layer can be, put immediately in a watch-glass, containing water. This will prevent the, peel from getting folded or getting dry., What do we do with this peel?, Let us take a glass slide, put a drop of, water on it and transfer a small piece, of the peel from the watch glass to the, slide. Make sure that the peel is, perfectly flat on the slide. A thin camel, hair paintbrush might be necessary to, help transfer the peel. Now we put a, drop of safranin solution on this piece, followed by a cover slip. Take care to, , LIFE, , avoid air bubbles while putting the, cover slip with the help of a mounting, needle. Ask your teacher for help. We, have prepared a temporary mount of, onion peel. We can observe this slide, under low power followed by high, powers of a compound microscope., , While examining a thin slice of cork, Robert, Hooke saw that the cork resembled the, structure of a honeycomb consisting of many, little compartments. Cork is a substance, which comes from the bark of a tree. This, was in the year 1665 when Hooke made this, chance observation through a self-designed, microscope. Robert Hooke called these boxes, cells. Cell is a Latin word for ‘a little room’., This may seem to be a very small and, insignificant incident but it is very important, in the history of science. This was the very first, time that someone had observed that living, things appear to consist of separate units. The, use of the word ‘cell’ to describe these units is, being used till this day in biology., Let us find out about cells., , •, , OF, , Fig. 5.1: Compound microscope, , What do we observe as we look through, the lens? Can we draw the structures that, we are able to see through the microscope,, on an observation sheet? Does it look like, Fig. 5.2?, , 2019-2020, , Nucleus, , Cells, , Fig. 5.2: Cells of an onion peel

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We can try preparing temporary mounts, of peels of onions of different sizes. What do, we observe? Do we see similar structures or, different structures?, What are these structures?, , More to know, , These structures look similar to each other., Together they form a big structure like an, onion bulb! We find from this activity that, onion bulbs of different sizes have similar, small structures visible under a microscope., The cells of the onion peel will all look the, same, regardless of the size of the onion they, came from., These small structures that we see are, the basic building units of the onion bulb., These structures are called cells. Not only, onions, but all organisms that we observe, around are made up of cells. However, there, are also single cells that live on their own., Cells were first discovered by, Robert Hooke in 1665. He observed, the cells in a cork slice with the help, of, a, primitive, microscope., Leeuwenhoek (1674), with the, improved microscope, discovered the, free living cells in pond water for the, first time. It was Robert Brown in, 1831 who discovered the nucleus in, the cell. Purkinje in 1839 coined the, ter m ‘protoplasm’ for the fluid, substance of the cell. The cell theory,, that all the plants and animals are, composed of cells and that the cell is, the basic unit of life, was presented, by two biologists, Schleiden (1838), and Schwann (1839). The cell theory, was further expanded by Virchow, (1855) by suggesting that all cells, arise from pre-existing cells. With the, discovery of the electron microscope, in 1940, it was possible to observe and, understand the complex structure of, the cell and its various organelles., , Chlamydomonas, Paramoecium and bacteria., These organisms are called unicellular, organisms (uni = single). On the other hand,, many cells group together in a single body, and assume different functions in it to form, various body parts in multicellular organisms, (multi = many) such as some fungi, plants, and animals. Can we find out names of some, more unicellular organisms?, Every multi-cellular organism has come, from a single cell. How? Cells divide to, produce cells of their own kind. All cells thus, come from pre-existing cells., , Activity ______________ 5.2, •, , •, , We can try preparing temporary, mounts of leaf peels, tip of roots of, onion or even peels of onions of different, sizes., After performing the above activity, let, us see what the answers to the following, questions would be:, (a) Do all cells look alike in terms of, shape and size?, (b) Do all cells look alike in structure?, (c) Could we find differences among, cells from different parts of a plant, body?, (d) What similarities could we find?, , Some organisms can also have cells of, different kinds. Look at the following picture., It depicts some cells from the human body., , Blood, cells, Smooth, muscle, cell, , Bone, cell, , The invention of magnifying lenses led to, the discovery of the microscopic world. It is, now known that a single cell may constitute, a whole organism as in Amoeba,, , Ovum, , Nerve Cell, , Fat cell, , Sperm, , Fig. 5.3: Various cells from the human body, , 58, , SCIENCE, , 2019-2020

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The shape and size of cells are related to, the specific function they perform. Some cells, like Amoeba have changing shapes. In some, cases the cell shape could be more or less, fixed and peculiar for a particular type of cell;, for example, nerve cells have a typical shape., Each living cell has the capacity to, perform certain basic functions that are, characteristic of all living forms. How does, a living cell perform these basic functions?, We know that there is a division of labour in, multicellular organisms such as human, beings. This means that different parts of, the human body perform different functions., The human body has a heart to pump blood,, a stomach to digest food and so on. Similarly,, division of labour is also seen within a single, cell. In fact, each such cell has got certain, specific components within it known as cell, organelles. Each kind of cell organelle, performs a special function, such as making, new material in the cell, clearing up the, waste material from the cell and so on. A, cell is able to live and perfor m all its, functions because of these organelles. These, organelles together constitute the basic unit, called the cell. It is interesting that all cells, are found to have the same organelles, no, matter what their function is or what, organism they are found in., , Q, , uestions, 1. Who discovered cells, and how?, 2. Why is the cell called the, structural and functional unit of, life?, , 5.2 What is a Cell Made Up of?, What is the Structural, Organisation of a Cell?, We saw above that the cell has special, components called organelles. How is a cell, organised?, If we study a cell under a microscope, we, would come across three features in almost, THE FUNDAMENTAL UNIT, , OF, , every cell; plasma membrane, nucleus and, cytoplasm. All activities inside the cell and, interactions of the cell with its environment, are possible due to these features. Let us see, how., , 5.2.1 P LASMA, , MEMBRANE, , OR, , CELL, , MEMBRANE, , This is the outermost covering of the cell that, separates the contents of the cell from its, external environment. The plasma membrane, allows or permits the entry and exit of some, materials in and out of the cell. It also, prevents movement of some other materials., The cell membrane, therefore, is called a, selectively permeable membrane., How does the movement of substances, take place into the cell? How do substances, move out of the cell?, Some substances like carbon dioxide or, oxygen can move across the cell membrane, by a process called diffusion. We have studied, the process of diffusion in earlier chapters., We saw that there is spontaneous movement, of a substance from a region of high, concentration to a region where its, concentration is low., Something similar to this happens in cells, when, for example, some substance like CO2, (which is cellular waste and requires to be, excreted out by the cell) accumulates in high, concentrations inside the cell. In the cell’s, external environment, the concentration of, CO2 is low as compared to that inside the, cell. As soon as there is a difference of, concentration of CO2 inside and outside a cell,, CO2 moves out of the cell, from a region of, high concentration, to a region of low, concentration outside the cell by the process, of diffusion. Similarly, O2 enters the cell by, the process of diffusion when the level or, concentration of O2 inside the cell decreases., Thus, diffusion plays an important role in, gaseous exchange between the cells as well, as the cell and its external environment., Water also obeys the law of diffusion. The, movement of water molecules through such a, selectively permeable membrane is called, 59, , LIFE, , 2019-2020

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osmosis. The movement of water across the, plasma membrane is also affected by the amount, of substance dissolved in water. Thus, osmosis, is the passage of water from a region of high, water concentration through a selectively, permeable membrane to a region of low water, concentration till equilibrium is reached., What will happen if we put an animal cell, or a plant cell into a solution of sugar or salt, in water?, One of the following three things could, happen:, 1. If the medium surrounding the cell has, a higher water concentration than the, cell, meaning that the outside solution, is very dilute, the cell will gain water, by osmosis. Such a solution is known, as a hypotonic solution., Water molecules are free to pass, across the cell membrane in both, directions, but more water will come, into the cell than will leave. The net, (overall) result is that water enters the, cell. The cell is likely to swell up., 2. If the medium has exactly the same, water concentration as the cell, there, will be no net movement of water, across the cell membrane. Such a, solution is known as an isotonic, solution., Water crosses the cell membrane, in both directions, but the amount, going in is the same as the amount, going out, so there is no overall, movement of water. The cell will stay, the same size., 3. If the medium has a lower, concentration of water than the cell,, meaning that it is a very concentrated, solution, the cell will lose water by, osmosis. Such a solution is known as, a hypertonic solution., Again, water crosses the cell, membrane in both directions, but this, time more water leaves the cell than, enters it. Therefore the cell will shrink., Thus, osmosis is a special case of diffusion, through a selectively permeable membrane., Now let us try out the following activity:, , Activity ______________ 5.3, (a), , (b), , Osmosis with an egg, Remove the shell of an egg by dissolving, it in dilute hydrochloric acid. The shell, is mostly calcium carbonate. A thin, outer skin now encloses the egg. Put, the egg in pure water and observe after, 5 minutes. What do we observe?, The egg swells because water passes, into it by osmosis., Place a similar de-shelled egg in a, concentrated salt solution and observe, for 5 minutes. The egg shrinks. Why?, Water passes out of the egg solution, into the salt solution because the salt, solution is more concentrated., , We can also try a similar activity with dried, raisins or apricots., , Activity ______________ 5.4, •, , Put dried raisins or apricots in plain, water and leave them for some time., Then place them into a concentrated, solution of sugar or salt. You will, observe the following:, (a) Each gains water and swells, when placed in water., (b) However, when placed in the, concentrated solution it loses, water, and consequently shrinks., , Unicellular freshwater organisms and, most plant cells tend to gain water through, osmosis. Absorption of water by plant roots, is also an example of osmosis., Thus, diffusion is important in exhange, of gases and water in the life of a cell. In, additions to this, the cell also obtains, nutrition from its environment. Different, molecules move in and out of the cell, through a type of transport requiring use, of energy., The plasma membrane is flexible and is, made up of organic molecules called lipids, and proteins. However, we can observe the, structure of the plasma membrane only, through an electron microscope., The flexibility of the cell membrane also, enables the cell to engulf in food and other, material from its external environment. Such, processes are known as endocytosis. Amoeba, acquires its food through such processes., , 60, , SCIENCE, , 2019-2020

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Activity ______________ 5.5, •, , Q, , Find out about electron microscopes, from resources in the school library or, through the internet. Discuss it with, your teacher., , uestions, 1. How do substances like CO2 and, water move in and out of the cell?, Discuss., 2. Why is the plasma membrane, called a selectively permeable, membrane?, , 5.2.3 NUCLEUS, , 5.2.2 CELL WALL, Plant cells, in addition to the plasma, membrane, have another rigid outer covering, called the cell wall. The cell wall lies outside, the plasma membrane. The plant cell wall is, mainly composed of cellulose. Cellulose is a, complex substance and provides structural, strength to plants., When a living plant cell loses water, through osmosis there is shrinkage or, contraction of the contents of the cell away, from the cell wall. This phenomenon is known, as plasmolysis. We can observe this, phenomenon by performing the following, activity:, , Activity ______________ 5.6, •, , •, , What do we infer from this activity? It, appears that only living cells, and not dead, cells, are able to absorb water by osmosis., Cell walls permit the cells of plants, fungi, and bacteria to withstand very dilute, (hypotonic) external media without bursting., In such media the cells tend to take up water, by osmosis. The cell swells, building up, pressure against the cell wall. The wall exerts, an equal pressure against the swollen cell., Because of their walls, such cells can, withstand much greater changes in the, surrounding medium than animal cells., , Remember the temporary mount of onion peel, we prepared? We had put iodine solution on, the peel. Why? What would we see if we tried, observing the peel without putting the iodine, solution? Try it and see what the difference, is. Further, when we put iodine solution on, the peel, did each cell get evenly coloured?, According to their chemical composition, dif ferent regions of cells get coloured, differentially. Some regions appear darker, than other regions. Apart from iodine solution, we could also use safranin solution or, methylene blue solution to stain the cells., We have observed cells from an onion; let, us now observe cells from our own body., , Mount the peel of a Rhoeo leaf in water, on a slide and examine cells under, the high power of a microscope. Note, the small green granules, called, chloroplasts. They contain a green, substance called chlorophyll. Put a, strong solution of sugar or salt on the, mounted leaf on the slide. Wait for a, minute and observe under a, microscope. What do we see?, Now place some Rhoeo leaves in boiling, water for a few minutes. This kills the, cells. Then mount one leaf on a slide, and observe it under a microscope. Put, a strong solution of sugar or salt on, the mounted leaf on the slide. Wait for, a minute and observe it again. What, do we find? Did plasmolysis occur now?, , THE FUNDAMENTAL UNIT, , OF, , Activity ______________ 5.7, •, , •, , Let us take a glass slide with a drop of, water on it. Using an ice-cream spoon, gently scrape the inside surface of the, cheek. Does any material get stuck on, the spoon? With the help of a needle, we can transfer this material and, spread it evenly on the glass slide kept, ready for this. To colour the material, we can put a drop of methylene blue, solution on it. Now the material is ready, for observation under microscope. Do, not forget to put a cover-slip on it!, What do we observe? What is the shape, of the cells we see? Draw it on the, observation sheet., 61, , LIFE, , 2019-2020

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•, , Was there a darkly coloured, spherical, or oval, dot-like structure near the, centre of each cell? This structure is, called nucleus. Were there similar, structures in onion peel cells?, , The nucleus has a double layered covering, called nuclear membrane. The nuclear, membrane has pores which allow the transfer, of material from inside the nucleus to its, outside, that is, to the cytoplasm (which we, will talk about in section 5.2.4)., The nucleus contains chromosomes,, which are visible as rod-shaped structures, only when the cell is about to divide., Chromosomes contain infor mation for, inheritance of characters from parents to next, generation in the form of DNA (Deoxyribo, Nucleic Acid) molecules. Chromosomes are, composed of DNA and protein. DNA molecules, contain the information necessary for, constructing and organising cells. Functional, segments of DNA are called genes. In a cell, which is not dividing, this DNA is present as, part of chromatin material. Chromatin, material is visible as entangled mass of thread, like structures. Whenever the cell is about to, divide, the chromatin material gets organised, into chromosomes., The nucleus plays a central role in cellular, reproduction, the process by which a single, cell divides and forms two new cells. It also, plays a crucial part, along with the, environment, in determining the way the cell, will develop and what form it will exhibit at, maturity, by directing the chemical activities, of the cell., In some organisms like bacteria, the, nuclear region of the cell may be poorly, defined due to the absence of a nuclear, membrane. Such an undefined nuclear region, containing only nucleic acids is called a, nucleoid. Such organisms, whose cells lack, a nuclear membrane, are called prokaryotes, (Pro = primitive or primary; karyote ≈ karyon, = nucleus). Organisms with cells having a, nuclear membrane are called eukaryotes., Prokaryotic cells (see Fig. 5.4) also lack, most of the other cytoplasmic organelles, , present in eukaryotic cells. Many of the, functions of such organelles are also, performed by poorly organised parts of the, cytoplasm (see section 5.2.4). The chlorophyll, in photosynthetic prokaryotic bacteria is, associated with membranous vesicles (bag, like structures) but not with plastids as in, eukaryotic cells (see section 5.2.5)., , Plasma, membrane, , Ribosomes, , Cell wall, , Nucleoid, , Fig. 5.4: Prokaryotic cell, , 5.2.4 CYTOPLASM, When we look at the temporary mounts of, onion peel as well as human cheek cells, we, can see a large region of each cell enclosed, by the cell membrane. This region takes up, very little stain. It is called the cytoplasm., The cytoplasm is the fluid content inside the, plasma membrane. It also contains many, specialised cell organelles. Each of these, organelles performs a specific function for the, cell., Cell organelles are enclosed by, membranes. In prokaryotes, beside the, absence of a defined nuclear region, the, membrane-bound cell organelles are also, absent. On the other hand, the eukaryotic, cells have nuclear membrane as well as, membrane-enclosed organelles., The significance of membranes can be, illustrated with the example of viruses., Viruses lack any membranes and hence do, not show characteristics of life until they enter, a living body and use its cell machinery to, multiply., , 62, , SCIENCE, , 2019-2020

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Q, , 5.2.5 (i) ENDOPLASMIC RETICULUM (ER), , uestion, 1. Fill in the gaps in the following, table illustrating differences, between prokaryotic and, eukaryotic cells., , Prokaryotic Cell, , Eukaryotic Cell, , 1. Size : generally, small ( 1-10 µm), 1 µm = 10–6 m, , 1. Size: generally, large ( 5-100 µm), , 2. Nuclear region:, _______________, _______________, and known as__, , 2. Nuclear region:, well defined and, surrounded by a, nuclear membrane, , 3. Chromosome:, single, , 3. More than one, chromosome, , 4. Membrane-bound 4. _______________, cell organelles, _______________, absent, _______________, , The endoplasmic reticulum (ER) is a large, network of membrane-bound tubes and, sheets. It looks like long tubules or round or, oblong bags (vesicles). The ER membrane is, similar in structure to the plasma membrane., There are two types of ER– rough endoplasmic, reticulum (RER) and smooth endoplasmic, reticulum (SER). RER looks rough under a, microscope because it has particles called, ribosomes attached to its surface. The, ribosomes, which are present in all active, cells, are the sites of protein manufacture., The manufactured proteins are then sent to, various places in the cell depending on need,, using the ER. The SER helps in the, manufacture of fat molecules, or lipids,, important for cell function. Some of these, proteins and lipids help in building the cell, membrane. This pr ocess is known as, membrane biogenesis. Some other proteins, and lipids function as enzymes and, hormones. Although the ER varies greatly in, appearance in different cells, it always forms a, network system., , 5.2.5 CELL ORGANELLES, Every cell has a membrane around it to keep, its own contents separate from the external, environment. Large and complex cells,, including cells from multicellular organisms,, need a lot of chemical activities to support, their complicated structure and function. To, keep these activities of different kinds, separate from each other, these cells use, membrane-bound little structures (or, ‘organelles’) within themselves. This is one of, the features of the eukaryotic cells that, distinguish them from prokaryotic cells. Some, of these organelles are visible only with an, electron microscope., We have talked about the nucleus in a, previous section. Some important examples, of cell organelles which we will discuss now, are: endoplasmic reticulum, Golgi apparatus,, lysosomes, mitochondria and plastids. They, are important because they carry out some, very crucial functions in cells., THE FUNDAMENTAL UNIT, , OF, , Fig. 5.5: Animal cell, , Thus, one function of the ER is to serve as, channels for the transport of materials, (especially proteins) between various regions, of the cytoplasm or between the cytoplasm, and the nucleus. The ER also functions as a, cytoplasmic framework providing a surface, 63, , LIFE, , 2019-2020

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for some of the biochemical activities of the, cell. In the liver cells of the group of animals, called vertebrates (see Chapter 7), SER plays, a crucial role in detoxifying many poisons and, drugs., , Fig. 5.6: Plant cell, , 5.2.5 (ii) GOLGI APPARATUS, The Golgi apparatus, first described by, Camillo Golgi, consists of a system of, membrane-bound vesicles (flattened sacs), arranged approximately parallel to each other, in stacks called cisterns. These membranes, often have connections with the membranes, of ER and therefore constitute another portion, of a complex cellular membrane system., The material synthesised near the ER is, packaged and dispatched to various targets, inside and outside the cell through the Golgi, apparatus. Its functions include the storage,, modification and packaging of products in, vesicles. In some cases, complex sugars may, be made from simple sugars in the Golgi, apparatus. The Golgi apparatus is also, involved in the formation of lysosomes [see, 5.2.5 (iii)]., , Camillo Golgi was born at, Corteno near Brescia in, 1843., He, studied, medicine at the University, of Pavia. After graduating, in 1865, he continued to, work in Pavia at the, Hospital of St. Matteo. At, that time most of his, investigations, were, concerned with the nervous system, In 1872, he accepted the post of Chief Medical Officer, at the Hospital for the Chronically Sick at, Abbiategrasso. He first started his, investigations into the nervous system in a, little kitchen of this hospital, which he had, converted into a laboratory. However, the, work of greatest importance, which Golgi, carried out was a revolutionary method of, staining individual nerve and cell structures., This method is referred to as the ‘black, reaction’. This method uses a weak solution, of silver nitrate and is particularly valuable, in tracing the processes and most delicate, ramifications of cells. All through his life,, he continued to work on these lines,, modifying and improving this technique., Golgi received the highest honours and, awards in recognition of his work. He shared, the Nobel prize in 1906 with Santiago, Ramony Cajal for their work on the structure, of the nervous system., , 5.2.5 (iii) LYSOSOMES, Structurally, lysosomes are membrane-bound, sacs filled with digestive enzymes. These, enzymes are made by RER. Lysosomes are a, kind of waste disposal system of the cell. These, help to keep the cell clean by digesting any, foreign material as well as worn-out cell, organelles. Foreign materials entering the cell,, such as bacteria or food, as well as old, organelles end up in the lysosomes, which, break complex substances into simpler, substances. Lysosomes are able to do this, because they contain powerful digestive, enzymes capable of breaking down all organic, material. During the disturbance in cellular, metabolism, for example, when the cell gets, , 64, , SCIENCE, , 2019-2020

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damaged, lysosomes may burst and the, enzymes digest their own cell. Therefore,, lysosomes are also known as the ‘suicide bags’, of a cell., , 5.2.5 (iv) MITOCHONDRIA, Mitochondria are known as the powerhouses, of the cell. Mitochondria have two membrane, coverings. The outer membrane is porous, while the inner membrane is deeply folded., These folds increase surface area for ATPgenerating chemical reactions. The energy, required for various chemical activities needed, for life is released by mitochondria in the form, of ATP (Adenosine triphopshate) molecules., ATP is known as the energy currency of the, cell. The body uses energy stored in ATP for, making new chemical compounds and for, mechanical work., Mitochondria are strange organelles in the, sense that they have their own DNA and, ribosomes. Therefore, mitochondria are able, to make some of their own proteins., , In plant cells vacuoles are full of cell sap, and provide turgidity and rigidity to the cell., Many substances of importance in the life of, the plant cell are stored in vacuoles. These, include amino acids, sugars, various organic, acids and some proteins. In single-celled, organisms like Amoeba, the food vacuole, contains the food items that the Amoeba has, consumed. In some unicellular organisms,, specialised vacuoles also play important roles, in expelling excess water and some wastes, from the cell., , Q, , 5.2.5 (V) PLASTIDS, Plastids are present only in plant cells. There, are two types of plastids – chromoplasts, (coloured plastids) and leucoplasts (white or, colourless plastids). Chromoplasts containing, the pigment chlorophyll are known as, chloroplasts. Chloroplasts are important for, photosynthesis in plants. Chloroplasts also, contain various yellow or orange pigments in, addition to chlorophyll. Leucoplasts are, primarily organelles in which materials such, as starch, oils and protein granules are stored., The internal organisation of the Chloroplast, consists of numerous membrane layers, embedded in a material called the stroma. These, are similar to mitochondria in external, structure. Like the mitochondria, plastids also, have their own DNA and ribosomes., , 5.2.5 (vi) VACUOLES, Vacuoles are storage sacs for solid or liquid, contents. Vacuoles are small sized in animal, cells while plant cells have very large vacuoles., The central vacuole of some plant cells may, occupy 50-90% of the cell volume., THE FUNDAMENTAL UNIT, , OF, , uestions, 1. Can you name the two, organelles we have studied that, contain their own genetic, material?, 2. If the organisation of a cell is, destroyed due to some physical, or chemical influence, what will, happen?, 3. Why are lysosomes known as, suicide bags?, 4. Where are proteins synthesised, inside the cell?, , Each cell thus acquires its structure and, ability to function because of the organisation, of its membrane and organelles in specific, ways. The cell thus has a basic structural, organisation. This helps the cells to perform, functions like respiration, obtaining nutrition,, and clearing of waste material, or forming new, proteins., Thus, the cell is the fundamental structural, unit of living organisms. It is also the basic, functional unit of life., Cell Division, New cells are formed in organisms in order to, grow, to replace old, dead and injured cells,, and to form gametes required for reproduction., The process by which new cells are made is, called cell division. There are two main types, of cell division: mitosis and meiosis., The process of cell division by which most, of the cells divide for growth is called mitosis., In this process, each cell called mother cell, 65, , LIFE, , 2019-2020

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Fig. 5.7: Mitosis, , Fig. 5.8: Meiosis, , divides to form two identical daughter cells, (Fig. 5.7). The daughter cells have the same, number of chromosomes as mother cell. It, helps in growth and repair of tissues in, organisms., Specific cells of reproductive organs or, tissues in animals and plants divide to form, gametes, which after fertilisation give rise to, , offspring. They divide by a different process, called meiosis which involves two consecutive, divisions. When a cell divides by meiosis it, produces four new cells instead of just two (Fig., 5.8). The new cells only have half the number, of chromosomes than that of the mother cells., Can you think as to why the chromosome, number has reduced to half in daughter cells?, , What, you have, learnt, •, •, •, , •, •, , •, , •, •, , •, , The fundamental organisational unit of life is the cell., Cells are enclosed by a plasma membrane composed of lipids, and proteins., The cell membrane is an active part of the cell. It regulates, the movement of materials between the ordered interior of, the cell and the outer environment., In plant cells, a cell wall composed mainly of cellulose is, located outside the cell membrane., The presence of the cell wall enables the cells of plants,, fungi and bacteria to exist in hypotonic media without, bursting., The nucleus in eukaryotes is separated from the cytoplasm, by double-layered membrane and it directs the life processes, of the cell., The ER functions both as a passageway for intracellular, transport and as a manufacturing surface., The Golgi apparatus consists of stacks of membrane-bound, vesicles that function in the storage, modification and, packaging of substances manufactured in the cell., Most plant cells have large membranous organelles called, plastids, which are of two types – chromoplasts and, leucoplasts., , 66, , SCIENCE, , 2019-2020

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•, •, •, , •, , •, , Chromoplasts that contain chlorophyll are called, chloroplasts and they perform photosynthesis., The primary function of leucoplasts is storage., Most mature plant cells have a large central vacuole that, helps to maintain the turgidity of the cell and stores, important substances including wastes., Prokaryotic cells have no membrane-bound organelles, their, chromosomes are composed of only nucleic acid, and they, have only very small ribosomes as organelles., Cells in organisms divide for growth of body, for repalcing, dead cells, and for forming gametes for reproduction., , Exercises, 1. Make a comparison and write down ways in which plant, cells are different from animal cells., 2. How is a prokaryotic cell different from a eukaryotic cell?, 3. What would happen if the plasma membrane ruptures or, breaks down?, 4. What would happen to the life of a cell if there was no Golgi, apparatus?, 5. Which organelle is known as the powerhouse of the cell? Why?, 6. Where do the lipids and proteins constituting the cell, membrane get synthesised?, 7. How does an Amoeba obtain its food?, 8. What is osmosis?, 9. Carry out the following osmosis experiment:, Take four peeled potato halves and scoos each one out to, make potato cups. One of these potato cups should be made, from a boiled potato. Put each potato cup in a trough, containing water. Now,, (a) Keep cup A empty, (b) Put one teaspoon sugar in cup B, (c) Put one teaspoon salt in cup C, (d) Put one teaspoon sugar in the boiled potato cup D., Keep these for two hours. Then observe the four potato cups, and answer the following:, (i) Explain why water gathers in the hollowed portion of, B and C., (ii) Why is potato A necessary for this experiment?, (iii) Explain why water does not gather in the hollowed out, portions of A and D., 10. Which type of cell division is required for growth and repair, of body and which type is involved in formation of gametes?, THE FUNDAMENTAL UNIT, , OF, , 67, , LIFE, , 2019-2020

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Chapter 6, TISSUES, From the last chapter, we recall that all living, organisms are made of cells. In unicellular, organisms, a single cell performs all basic, functions. For example, in Amoeba, a single, cell carries out movement, intake of food,, gaseous exchange and excretion. But in multicellular organisms there are millions of cells., Most of these cells are specialised to carry out, specific functions. Each specialised function, is taken up by a different group of cells. Since, these cells carry out only a particular function,, they do it very efficiently. In human beings,, muscle cells contract and relax to cause, movement, nerve cells carry messages, blood, flows to transport oxygen, food, hormones and, waste material and so on. In plants, vascular, tissues conduct food and water from one part, of the plant to other parts. So, multi-cellular, organisms show division of labour. Cells, specialising in one function are often grouped, together in the body. This means that a, particular function is carried out by a cluster, of cells at a definite place in the body. This, cluster of cells, called a tissue, is arranged and, designed so as to give the highest possible, efficiency of function. Blood, phloem and, muscle are all examples of tissues., A group of cells that are similar in structure, and/or work together to achieve a particular, function forms a tissue., , 6.1 Are Plants and Animals Made, of Same Types of Tissues?, Let us compare their structure and functions., Do plants and animals have the same, structure? Do they both perform similar, functions?, , There are noticeable differences between, the two. Plants are stationary or fixed – they, don’t move. Since they have to be upright, they, have a large quantity of supportive tissue. The, supportive tissue generally has dead cells., Animals on the other hand move around, in search of food, mates and shelter. They, consume more energy as compared to plants., Most of the tissues they contain are living., Another difference between animals and, plants is in the pattern of growth. The growth, in plants is limited to certain regions, while this, is not so in animals. There are some tissues in, plants that divide throughout their life. These, tissues are localised in certain regions. Based, on the dividing capacity of the tissues, various, plant tissues can be classified as growing or, meristematic tissue and permanent tissue. Cell, growth in animals is more uniform. So, there, is no such demarcation of dividing and nondividing regions in animals., The structural organisation of organs and, organ systems is far more specialised and, localised in complex animals than even in very, complex plants. This fundamental difference, reflects the different modes of life pursued by, these two major groups of organisms,, particularly in their different feeding methods., Also, they are differently adapted for a, sedentary existence on one hand (plants) and, active locomotion on the other (animals),, contributing to this difference in organ system, design., It is with reference to these complex animal, and plant bodies that we will now talk about, the concept of tissues in some detail., , 2019-2020

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Q, , •, , uestions, 1. What is a tissue?, 2. What is the utility of tissues in, multi-cellular organisms?, , 6.2 Plant Tissues, 6.2.1 MERISTEMATIC, , From the above observations, answer, the following questions:, 1. Which of the two onions has longer, roots? Why?, 2. Do the roots continue growing, even after we have removed their, tips?, 3. Why would the tips stop growing, in jar 2 after we cut them?, , The growth of plants occurs only in certain, specific regions. This is because the dividing, tissue, also known as meristematic tissue, is, located only at these points. Depending on, the region where they are present,, meristematic tissues are classified as apical,, lateral and intercalary (Fig. 6.2). New cells, produced by meristem are initially like those, of meristem itself, but as they grow and, mature, their characteristics slowly change and, they become differentiated as components of, other tissues., , TISSUE, , Apical meristem, , Jar 1, , Jar 2, , Fig. 6.1: Growth of roots in onion bulbs, Intercalary meristem, , Activity ______________ 6.1, •, •, , •, •, •, , Take two glass jars and fill them with, water., Now, take two onion bulbs and place, one on each jar, as shown in, Fig. 6.1., Observe the growth of roots in both, the bulbs for a few days., Measure the length of roots on day 1,, 2 and 3., On day 4, cut the root tips of the onion, bulb in jar 2 by about 1 cm. After this,, observe the growth of roots in both the, jars and measure their lengths each, day for five more days and record the, observations in tables, like the table, below:, , Length, Jar 1, Jar 2, , Day 1, , Day 2, , Day 3, , Day 4 Day 5, , Lateral meristem, , Fig. 6.2: Location of meristematic tissue in plant body, , Apical meristem is present at the growing, tips of stems and roots and increases the, length of the stem and the root. The girth of, the stem or root increases due to lateral, meristem (cambium). Intercalary meristem, seen in some plants is located near the node., 69, , TISSUES, , 2019-2020

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Cells of meristematic tissue are very active,, they have dense cytoplasm, thin cellulose walls, and prominent nuclei. They lack vacuoles. Can, we think why they would lack vacuoles? (You, might want to refer to the functions of vacuoles, in the chapter on cells.), , 6.2.2 PERMANENT, , TISSUE, , What happens to the cells formed by, meristematic tissue? They take up a specific, role and lose the ability to divide. As a result,, they form a permanent tissue. This process, of taking up a permanent shape, size, and a, function is called differentiation. Differentiation, leads to the development of various types of, permanent tissues., , •, , 3. Can we think of reasons why there, would be so many types of cells?, We can also try to cut sections of plant, roots. We can even try cutting sections, of root and stem of different plants., , 6.2.2 (i) SIMPLE PERMANENT TISSUE, A few layers of cells beneath the epidermis are, generally simple permanent tissue., Parenchyma is the most common simple, permanent tissue. It consists of relatively, unspecialised cells with thin cell walls. They, are living cells. They are usually loosely, arranged, thus large spaces between cells, (intercellular spaces) are found in this tissue, (Fig. 6.4 a). This tissue generally stores food., , Cuticle, Epidermis, Collenchyma, Parenchyma, , Phloem, , Xylem, Vascular bundle, , Fig. 6.3: Section of a stem, , Activity ______________ 6.2, •, , Take a plant stem and with the help, of your teacher cut into very thin slices, or sections., Now, stain the slices with safranin., Place one neatly cut section on a slide,, and put a drop of glycerine., Cover with a cover-slip and observe, under a microscope. Observe the, various types of cells and their, arrangement. Compare it with Fig. 6.3., Now, answer the following on the, basis of your observation:, 1. Are all cells similar in structure?, 2. How many types of cells can, be seen?, , •, , •, , •, , In some situations, it contains chlorophyll and, performs photosynthesis, and then it is called, chlorenchyma. In aquatic plants, large air, cavities are present in parenchyma to help, them float. Such a parenchyma type is called, aerenchyma., The flexibility in plants is due to another, permanent tissue, collenchyma. It allows, bending of various parts of a plant like tendrils, and stems of climbers without breaking. It, also provides mechanical support. We can find, this tissue in leaf stalks below the epidermis., The cells of this tissue are living, elongated, and irregularly thickened at the, corners. There is very little intercellular space, (Fig. 6.4 b)., , 70, , SCIENCE, , 2019-2020

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Intercellular spaces, Wall thickenings, Nucleus, , Thick lignified, walls, , Vacuole, Cell wall, , a, , Narrow lumen, Lignified, thick wall, , c (ii), , c (i), , b, , Fig. 6.4: Various types of simple tissues: (a) Parenchyma (b) Collenchyma (c) Sclerenchyma (i) transverse section,, (ii) longitudinal section., , Yet another type of permanent tissue is, sclerenchyma. It is the tissue which makes the, plant hard and stiff. We have seen the husk of, a coconut. It is made of sclerenchymatous, tissue. The cells of this tissue are dead. They, are long and narrow as the walls are thickened, due to lignin. Often these walls are so thick, that there is no internal space inside the cell, (Fig. 6.4 c). This tissue is present in stems,, around vascular bundles, in the veins of leaves, and in the hard covering of seeds and nuts. It, provides strength to the plant parts., , Activity ______________ 6.3, •, •, •, , •, •, •, , •, , Take a freshly plucked leaf of Rhoeo., Stretch and break it by applying, pressure., While breaking it, keep it stretched, gently so that some peel or skin, projects out from the cut., Remove this peel and put it in a petri, dish filled with water., Add a few drops of safranin., Wait for a couple of minutes and then, transfer it onto a slide. Gently place, a cover slip over it., Observe under microscope., , What you observe is the outermost layer, of cells, called epidermis. The epidermis is, usually made of a single layer of cells. In some, plants living in very dry habitats, the epidermis, may be thicker since protection against water, loss is critical. The entire surface of a plant has, an outer covering epidermis. It protects all the, parts of the plant. Epidermal cells on the aerial, , Guard, cells, Stoma, Epidermal, cell, (a), , Guard, cell, (b), , Fig. 6.5: Guard cells and epidermal cells: (a) lateral, view, (b) surface view, , parts of the plant often secrete a waxy, waterresistant layer on their outer surface. This aids, in protection against loss of water, mechanical, injury and invasion by parasitic fungi. Since, it has a protective role to play, cells of, epidermal tissue form a continuous layer, without intercellular spaces. Most epidermal, cells are relatively flat. Often their outer and, side walls are thicker than the inner wall., We can observe small pores here and, there in the epidermis of the leaf. These pores, are called stomata (Fig. 6.5). Stomata are, enclosed by two kidney-shaped cells, called guard cells. They are necessary for, exchanging gases with the atmosphere., Transpiration (loss of water in the form of, water vapour) also takes place through, stomata., 71, , TISSUES, , 2019-2020

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Recall which gas is required for, photosynthesis., Find out the role of transpiration in plants., Epidermal cells of the roots, whose function, is water absorption, commonly bear long hairlike parts that greatly increase the total, absorptive surface area., In some plants like desert plants,, epidermis has a thick waxy coating of cutin, (chemical substance with waterproof quality), on its outer surface. Can we think of a reason, for this?, Is the outer layer of a branch of a tree, different from the outer layer of a young stem?, As plants grow older, the outer protective, tissue undergoes certain changes. A strip of, secondary meristem located in the cortex forms, layers of cells which constitute the cork. Cells, of cork are dead and compactly arranged, without intercellular spaces (Fig. 6.6). They, also have a substance called suberin in their, walls that makes them impervious to gases, and water., Cork cells, , is a distinctive feature of the complex plants,, one that has made possible their survival in, the terrestrial environment. In Fig. 6.3 showing, a section of stem, can you see different types, of cells in the vascular bundle?, Xylem consists of tracheids, vessels, xylem, parenchyma (Fig. 6.7 a,b,c) and xylem fibres., Tracheids and vessels have thick walls, and, many are dead cells when mature. Tracheids, and vessels are tubular structures. This allows, them to transport water and minerals, vertically. The parenchyma stores food. Xylem, fibres are mainly supportive in function., Phloem is made up of five types of cells:, sieve cells, sieve tubes, companion cells,, phloem fibres and the phloem parenchyma, [Fig. 6.7 (d)]. Sieve tubes are tubular cells with, perforated walls. Phloem transports food from, leaves to other parts of the plant. Except, phloem fibres, other phloem cells are living cells., , Nucleus, , Ruptured epidermis, Pit, , Pits, Cytoplasm, , (a) Tracheid, , Fig. 6.6: Protective tissue, , 6.2.2 (ii) COMPLEX PERMANENT TISSUE, , (b) Vessel, , (c) Xylem parenchyma, , Sieve plate, , The different types of tissues we have discussed, until now are all made of one type of cells,, which look like each other. Such tissues are, called simple permanent tissue. Yet another, type of permanent tissue is complex tissue., Complex tissues are made of more than one, type of cells. All these cells coordinate to, perform a common function. Xylem and, phloem are examples of such complex tissues., They are both conducting tissues and, constitute a vascular bundle. Vascular tissue, 72, , Sieve tube, , Phloem, parenchyma, Companion cell, , (d) Section of phloem, , Fig. 6.7: Types of complex tissue, SCIENCE, , 2019-2020

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Q, , uestions, 1. Name types of simple tissues., 2. Where is apical meristem found?, 3. Which tissue makes up the husk, of coconut?, 4. What are the constituents of, phloem?, , 6.3 Animal Tissues, When we breathe we can actually feel the, movement of our chest. How do these body, parts move? For this we have specialised cells, called muscle cells (Fig. 6.8). The contraction, and relaxation of these cells result in, movement., , During breathing we inhale oxygen. Where, does this oxygen go? It is absorbed in the lungs, and then is transported to all the body cells, through blood. Why would cells need oxygen?, The functions of mitochondria we studied, earlier provide a clue to this question. Blood, flows and carries various substances from one, part of the body to the other. For example, it, carries oxygen and food to all cells. It also, collects wastes from all parts of the body and, carries them to the liver and kidney for, disposal., Blood and muscles are both examples of, tissues found in our body. On the basis of the, functions they perform we can think of different, types of animal tissues, such as epithelial, tissue, connective tissue, muscular tissue and, nervous tissue. Blood is a type of connective, tissue, and muscle forms muscular tissue., , 6.3.1 EPITHELIAL, , Smooth muscle fibres, , Nucleus, Smooth muscle fibre, (Cell), , Fig. 6.8: Location of muscle fibres, , TISSUE, , The covering or protective tissues in the animal, body are epithelial tissues. Epithelium covers, most organs and cavities within the body. It, also forms a barrier to keep different body, systems separate. The skin, the lining of the, mouth, the lining of blood vessels, lung alveoli, and kidney tubules are all made of epithelial, tissue. Epithelial tissue cells are tightly packed, and form a continuous sheet. They have only, a small amount of cementing material between, them and almost no intercellular spaces., Obviously, anything entering or leaving the, body must cross at least one layer of, epithelium. As a result, the permeability of the, cells of various epithelia play an important role, in regulating the exchange of materials, between the body and the external, environment and also between different parts, of the body. Regardless of the type, all, epithelium is usually separated from the, underlying tissue by an extracellular fibrous, basement membrane., Different epithelia (Fig. 6.9) show differing, structures that correlate with their unique, functions. For example, in cells lining blood, vessels or lung alveoli, where transportation, of substances occurs through a selectively, permeable surface, there is a simple flat kind, 73, , TISSUES, , 2019-2020

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of epithelium. This is called the simple, squamous epithelium (squama means scale, , (a) Squamous, , (b) Stratified squamous, , of skin). Simple squamous epithelial cells are, extremely thin and flat and form a delicate, lining. The oesophagus and the lining of the, mouth are also covered with squamous, epithelium. The skin, which protects the body,, is also made of squamous epithelium. Skin, epithelial cells are arranged in many layers to, prevent wear and tear. Since they are arranged, in a pattern of layers, the epithelium is called, stratified squamous epithelium., Where absorption and secretion occur, as, in the inner lining of the intestine, tall epithelial, cells are present. This columnar (meaning, ‘pillar-like’) epithelium facilitates movement, across the epithelial barrier. In the respiratory, tract, the columnar epithelial tissue also has, cilia, which are hair-like projections on the, outer surfaces of epithelial cells. These cilia can, move, and their movement pushes the mucus, forward to clear it. This type of epithelium is, thus ciliated columnar epithelium., Cuboidal epithelium (with cube-shaped, cells) forms the lining of kidney tubules and, ducts of salivary glands, where it provides, mechanical support. Epithelial cells often, acquire additional specialisation as gland cells,, which can secrete substances at the epithelial, surface. Sometimes a portion of the epithelial, tissue folds inward, and a multicellular gland, is formed. This is glandular epithelium., , 6.3.2 CONNECTIVE TISSUE, (c) Cuboidal, , Blood is a type of connective tissue. Why would, it be called ‘connective’ tissue? A clue is, provided in the introduction of this chapter!, Now, let us look at this type of tissue in some, more detail. The cells of connective tissue are, loosely spaced and embedded in an, intercellular matrix (Fig. 6.10). The matrix may, be jelly like, fluid, dense or rigid. The nature, of matrix differs in concordance with the, function of the particular connective tissue., , Activity 6.4, (d) Columnar (Ciliated), , Take a drop of blood on a slide and, observe different cells present in it, under a microscope., , Fig. 6.9: Different types of epithelial tissues, , 74, , SCIENCE, , 2019-2020

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Cytoplasm, Nucleus, , Different white, blood corpuscles, , Neutrophil Eosinophil Basophil, (polynuclear, leucocyte), , Lymphocyte Monocyte, , Platelets, , (a), Haversian canal, (contains blood vessels, and nerve fibres), , Chondrocyte, Hyaline matrix, , (b), , Canaliculus (contains, slender process of bone, cell or osteocyte), , (c), , Red blood, corpuscle, Reticular fibre, , Fibroblast, , Macrophage, , Collagen fibre, , Mast cell, (d), Fat droplet, , Plasma cell, Nucleus, , Blood has a fluid (liquid) matrix called, plasma, in which red blood corpuscles (RBCs),, white blood corpuscles (WBCs) and platelets, are suspended. The plasma contains proteins,, salts and hormones. Blood flows and, transports gases, digested food, hormones, and waste materials to different parts of the, body., Bone is another example of a connective, tissue. It forms the framework that supports, the body. It also anchors the muscles and, supports the main organs of the body. It is a, strong and nonflexible tissue (what would be, the advantage of these properties for bone, functions?). Bone cells are embedded in a, hard matrix that is composed of calcium and, phosphorus compounds., Two bones can be connected to each other, by another type of connective tissue called the, ligament. This tissue is very elastic. It has, considerable strength. Ligaments contain, very little matrix and connect bones with, bones. Tendons connect muscles to bones and, are another type of connective tissue. Tendons, are fibrous tissue with great strength but, limited flexibility., Another type of connective tissue,, cartilage, has widely spaced cells. The solid, matrix is composed of proteins and sugars., Cartilage smoothens bone surfaces at joints, and is also present in the nose, ear, trachea, and larynx. We can fold the cartilage of the ears,, but we cannot bend the bones in our arms., Think of how the two tissues are different!, Areolar connective tissue is found between, the skin and muscles, around blood vessels, and nerves and in the bone marrow. It fills, the space inside the organs, supports internal, organs and helps in repair of tissues., Where are fats stored in our body? Fatstoring adipose tissue is found below the skin, and between internal organs. The cells of this, tissue are filled with fat globules. Storage of, fats also lets it act as an insulator., , Adipocyte, , 6.3.3 MUSCULAR, , (e), , Fig. 6.10: Types of connective tissues: (a) types of blood, cells, (b) compact bone, (c) hyaline cartilage,, (d) areolar tissue, (e) adipose tissue, , TISSUE, , Muscular tissue consists of elongated cells,, also called muscle fibres. This tissue is, responsible for movement in our body., 75, , TISSUES, , 2019-2020

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Muscles contain special proteins called, contractile proteins, which contract and relax, to cause movement., Nuclei, Striations, , (a), , Spindle shaped, muscle cell, , Nucleus, (b), , Striations, , [Fig. 6.11(a)]. These muscles are also called, skeletal muscles as they are mostly attached, to bones and help in body movement. Under, the microscope, these muscles show alternate, light and dark bands or striations when, stained appropriately. As a result, they are, also called striated muscles. The cells of this, tissue are long, cylindrical, unbranched and, multinucleate (having many nuclei)., The movement of food in the alimentary, canal or the contraction and relaxation of blood, vessels are involuntary movements. We cannot, really start them or stop them simply by, wanting to do so! Smooth muscles [Fig., 6.11(b)] or involuntary muscles control such, movements. They are also found in the iris of, the eye, in ureters and in the bronchi of the, lungs. The cells are long with pointed ends, (spindle-shaped) and uninucleate (having a, single nucleus). They are also called unstriated, muscles – why would they be called that?, The muscles of the heart show rhythmic, contraction and relaxation throughout life., These involuntary muscles are called cardiac, muscles [Fig. 6.11(c)]. Heart muscle cells are, cylindrical, branched and uninucleate., , Activity 6.5, Compare the structures of different, types of muscular tissues. Note down, their shape, number of nuclei and, position of nuclei within the cell in, the Table 6.1., , Nuclei, , Table 6.1:, Features, , Striated, , Smooth, , Cardiac, , Shape, Number of nuclei, (c), , Position of nuclei, , Fig. 6.11: Types of muscles fibres: (a) striated muscle,, (b) smooth muscle, (c) cardiac muscle, , We can move some muscles by conscious, will. Muscles present in our limbs move when, we want them to, and stop when we so decide., Such muscles are called voluntary muscles, , 6.3.4 NERVOUS, , TISSUE, , All cells possess the ability to respond to, stimuli. However, cells of the nervous tissue, are highly specialised for being stimulated and, , 76, , SCIENCE, , 2019-2020

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then transmitting the stimulus very rapidly, from one place to another within the body. The, brain, spinal cord and nerves are all composed, of the nervous tissue. The cells of this tissue, are called nerve cells or neurons. A neuron, consists of a cell body with a nucleus and, cytoplasm, from which long thin hair-like, parts arise (Fig. 6.12). Usually each neuron, has a single long part (process), called the, axon, and many short, branched parts, Nucleus, Dendrite, , Axon, , Nerve ending, , Cell body, , Fig. 6.12: Neuron-unit of nervous tissue, , (processes) called dendrites. An individual, nerve cell may be up to a metre long. Many, nerve fibres bound together by connective, tissue make up a nerve., The signal that passes along the nerve fibre, is called a nerve impulse. Nerve impulses allow, us to move our muscles when we want to. The, functional combination of nerve and muscle, tissue is fundamental to most animals. This, combination enables animals to move rapidly, in response to stimuli., , Q, , uestions, 1. Name the tissue responsible for, movement in our body., 2. What does a neuron look like?, 3. Give three features of cardiac, muscles., 4. What are the functions of, areolar tissue?, , What, you have, learnt, •, , Tissue is a group of cells similar in structure and function., , •, , Plant tissues are of two main types – meristematic and, permanent., , •, , Meristematic tissue is the dividing tissue present in the, growing regions of the plant., , •, , Permanent tissues are derived from meristematic tissue once, they lose the ability to divide. They are classified as simple, and complex tissues., , •, , Parenchyma, collenchyma and sclerenchyma are three types, of simple tissues. Xylem and phloem are types of complex, tissues., , •, , Animal tissues can be epithelial, connective, muscular and, nervous tissue., , •, , Depending on shape and function, epithelial tissue is, classified as squamous, cuboidal, columnar, ciliated and, glandular., , 77, , TISSUES, , 2019-2020

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•, , The different types of connective tissues in our body include, areolar tissue, adipose tissue, bone, tendon, ligament,, cartilage and blood., , •, , Striated, unstriated and cardiac are three types of muscle, tissues., , •, , Nervous tissue is made of neurons that receive and conduct, impulses., , Exercises, 1. Define the term “tissue”., 2. How many types of elements together make up the xylem, tissue? Name them., 3. How are simple tissues different from complex tissues in, plants?, 4. Differentiate between parenchyma, collenchyma and, sclerenchyma on the basis of their cell wall., 5. What are the functions of the stomata?, 6. Diagrammatically show the difference between the three, types of muscle fibres., 7. What is the specific function of the cardiac muscle?, 8. Differentiate between striated, unstriated and cardiac, muscles on the basis of their structure and site/location in, the body., 9. Draw a labelled diagram of a neuron., 10. Name the following., (a) Tissue that forms the inner lining of our mouth., (b) Tissue that connects muscle to bone in humans., (c) Tissue that transports food in plants., (d) Tissue that stores fat in our body., (e) Connective tissue with a fluid matrix., (f) Tissue present in the brain., 11. Identify the type of tissue in the following: skin, bark of, tree, bone, lining of kidney tubule, vascular bundle., , 78, , SCIENCE, , 2019-2020

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12., 13., 14., 15., , Name the regions in which parenchyma tissue is present., What is the role of epidermis in plants?, How does the cork act as a protective tissue?, Complete the following chart:, , 79, , TISSUES, , 2019-2020

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Chapter 7, DIVERSITY, , IN, , LIVING ORGANISMS, , Have you ever thought of the multitude of, life-forms that surround us? Each organism, is different from the other to a lesser or greater, extent. For instance, consider yourself and a, friend., •, Are you both of the same height?, •, Does your nose look exactly like your, friend’s nose?, •, Is your hand-span the same as your, friend’s?, However, if we were to compare ourselves, and our friends with a monkey, what would, we say? Obviously, we and our friends have, a lot in common when we compare ourselves, with a monkey. But suppose we were to add, a cow to the comparison? We would then, think that the monkey has a lot more in, common with us than with the cow., , Activity ______________ 7.1, •, , •, •, •, •, , We have heard of ‘desi’ cows and Jersey, cows., Does a desi cow look like a Jersey cow?, Do all desi cows look alike?, Will we be able to identify a Jersey cow, in a crowd of desi cows that don’t look, like each other?, What is the basis of our identification?, , In this activity, we had to decide which, characteristics were more important in, forming the desired category. Hence, we were, also deciding which characteristics could be, ignored., Now, think of all the different forms in, which life occurs on earth. On one hand we, have microscopic bacteria of a few micrometre, in size. While on the other hand we have blue, whale and red wood trees of California, of approximate sizes of 30 metres and, 100 metres respectively. Some pine trees live, , for thousands of years while insects like, mosquitoes die within a few days. Life also, ranges from colourless or even transparent, worms to brightly coloured birds and flowers., This bewildering variety of life around us, has evolved on the earth over millions of, years. However, we do not have more than a, tiny fraction of this time to try and, understand all these living organisms, so we, cannot look at them one by one. Instead, we, look for similarities among the organisms,, which will allow us to put them into different, classes and then study different classes or, groups as a whole., In order to make relevant groups to study, the variety of life forms, we need to decide, which characteristics decide more, fundamental differences among organisms., This would create the main broad groups of, organisms. Within these groups, smaller subgroups will be decided by less important, characteristics., , Q, , uestions, 1. Why do we classify organisms?, 2. Give three examples of the range, of variations that you see in lifeforms around you., , 7.1 What is the Basis of, Classification?, Attempts at classifying living things into, groups have been made since time, immemorial. Greek thinker Aristotle classified, animals according to whether they lived on, , 2019-2020

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land, in water or in the air. This is a very, simple way of looking at life, but misleading, too. For example, animals that live in the sea, include corals, whales, octopuses, starfish, and sharks. We can immediately see that, these are very different from each other in, numerous ways. In fact, habitat is the only, point they share in common. This is not an, appropriate way of making groups of, organisms to study and think about., We therefore need to decide which, characteristics to be used as the basis for, making the broadest divisions. Then we will, have to pick the next set of characteristics, for making sub-groups within these divisions., This process of classification within each, group can then continue using new, characteristics each time., Before we go on, we need to think about, what is meant by ‘characteristics’. When we, are trying to classify a diverse group of, organisms, we need to find ways in which, some of them are similar enough to be, thought of together. These ‘ways’, in fact, are, details of appearance or behaviour, in other, words, form and function., What we mean by a characteristic is a, particular feature or a particular function. That, most of us have five fingers on each hand is, thus a characteristic. That we can run, but the, banyan tree cannot, is also a characteristic., Now, to understand how some, characteristics are decided as being more, fundamental than others, let us consider how, a stone wall is built. The stones used will have, different shapes and sizes. The stones at the, top of the wall would not influence the choice, of stones that come below them. On the other, hand, the shapes and sizes of stones in the, lowermost layer will decide the shape and size, of the next layer and so on., The stones in the lowermost layer are like, the characteristics that decide the broadest, divisions among living organisms. They are, independent of any other characteristics in, their effects on the form and function of the, organism. The characteristics in the next level, would be dependent on the previous one and, would decide the variety in the next level. In, this way, we can build up a whole hierarchy, D IVERSITY, , IN, , of mutually related characteristics to be used, for classification., Now-a-days, we look at many inter-related, characteristics starting from the nature of the, cell in order to classify all living organisms., What are some concrete examples of such, characteristics used for a hierarchical, classification?, •, A eukaryotic cell has membrane-bound, organelles, including a nucleus, which, allow cellular processes to be carried out, efficiently in isolation from each other., Therefore, organisms which do not have, a clearly demarcated nucleus and other, organelles would need to have their, biochemical pathways organised in very, different ways. This would have an effect, on every aspect of cell design. Further,, nucleated cells would have the capacity, to participate in making a multicellular, organism because they can take up, specialised functions. Therefore, nucleus, can be a basic characteristic of, classification., •, Do the cells occur singly or are they, grouped together and do they live as an, indivisible group? Cells that group, together to form a single organism use, the principle of division of labour. In such, a body design, all cells would not be, identical. Instead, groups of cells will, carry out specialised functions. This, makes a very basic distinction in the, body designs of organisms. As a result,, an Amoeba and a worm are very different, in their body design., •, Do organisms produce their own food, through the process of photosynthesis?, Being able to produce one’s own, food versus having to get food from, outside would make very different, body designs., •, Of the organisms that perform, photosynthesis (plants), what is the level, of organisation of their body?, •, Of the animals, how does the individual’s, body develop and organise its different, parts, and what are the specialised, organs found for different functions?, , L IVING O RGANISMS, , 81, , 2019-2020

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We can see that, even in these few questions, that we have asked, a hierarchy is developing., The characteristics of body design used for, classification of plants will be very different, from those important for classifying animals., This is because the basic designs are different,, based on the need to make their own food, (plants), or acquire it (animals). Therefore,, these design features (having a skeleton, for, example) are to be used to make sub-groups,, rather than making broad groups., , Q, , uestions, 1. Which do you think is a more basic, characteristic for classifying, organisms?, (a) the place where they live., (b) the kind of cells they are, made of. Why?, 2. What is the primary characteristic, on which the broad division of, organisms is made?, 3. On what bases are plants and, animals put into different, categories?, , When we connect this idea of evolution to, classification, we will find some groups of, organisms which have ancient body designs, that have not changed very much. We will, also find other groups of organisms that have, acquired their particular body designs, relatively recently. Those in the first group, are frequently referred to as ‘primitive’ or ‘lower’, organisms, while those in the second group, are called ‘advanced’ or ‘higher’ organisms. In, reality, these terms are not quite correct since, they do not properly relate to the differences., All that we can say is that some are ‘older’, organisms, while some are ‘younger’, organisms. Since there is a possibility that, complexity in design will increase over, evolutionary time, it may not be wrong to say, that older organisms are simpler, while, younger organisms are more complex., , All living things are identified and categorised, on the basis of their body design in form and, function. Some characteristics are likely to, make more wide-ranging changes in body, design than others. There is a role of time in, this as well. So, once a certain body design, comes into existence, it will shape the effects, of all other subsequent design changes,, simply because it already exists. In other, words, characteristics that came into, existence earlier are likely to be more basic, than characteristics that have come into, existence later., This means that the classification of life, forms will be closely related to their evolution., What is evolution? Most life forms that we, see today have arisen by an accumulation of, changes in body design that allow the, organism possessing them to survive better., Charles Darwin first described this idea of, evolution in 1859 in his book, The Origin of, Species., , More to know, , 7.2 Classification and Evolution, , Biodiversity means the diversity of life, forms. It is a word commonly used to, refer to the variety of life forms found, in a particular region. Diverse life forms, share the environment, and are, affected by each other too. As a result,, a stable community of different species, comes into existence. Humans have, played their own part in recent times, in changing the balance of such, communities. Of course, the diversity, in such communities is affected by, particular characteristics of land,, water, climate and so on. Rough, estimates state that there are about ten, million species on the planet, although, we actually know only one or two, millions of them. The warm and humid, tropical regions of the earth, between, the tropic of Cancer and the tropic of, Capricorn, are rich in diversity of plant, and animal life. This is called the region, of megadiversity. Of the biodiversity, on the planet, more than half is, concentrated in a few countries —, Brazil, Colombia, Ecuador, Peru,, Mexico,, Zaire,, Madagascar,, Australia, China, India, Indonesia and, Malaysia., SCIENCE, , 82, , 2019-2020

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Q, , 7.3.1 MONERA, , uestions, 1. Which organisms are called, primitive and how are they, different from the so-called, advanced organisms?, 2. Will advanced organisms be the, same as complex organisms?, Why?, , 7.3 The Hierarchy of ClassificationGroups, Biologists, such as Ernst Haeckel (1894),, Robert Whittaker (1969) and Carl Woese, (1977) have tried to classify all living, organisms into broad categories, called, kingdoms. The classification Whittaker, proposed has five kingdoms: Monera,, Protista, Fungi, Plantae and Animalia, and, is widely used. These groups are formed on, the basis of their cell structure, mode and, source of nutrition and body organisation., The modification Woese introduced by, dividing the Monera into Archaebacteria (or, Archaea) and Eubacteria (or Bacteria) is also, in use., Further classification is done by naming, the sub-groups at various levels as given in, the following scheme:, Kingdom, Phylum (for animals) / Division (for plants), Class, Order, Family, Genus, Species, Thus, by separating organisms on the, basis of a hierarchy of characteristics into, smaller and smaller groups, we arrive at the, basic unit of classification, which is a, ‘species’. So what organisms can be said to, belong to the same species? Broadly, a species, includes all organisms that are similar, enough to breed and perpetuate., The important characteristics of the five, kingdoms of Whittaker are as follows:, D IVERSITY, , IN, , These organisms do not have a defined, nucleus or organelles, nor do any of them, show multi-cellular body designs. On the, other hand, they show diversity based on, many other characteristics. Some of them, have cell walls while some do not. Of course,, having or not having a cell wall has very, different effects on body design here from, having or not having a cell wall in multicellular organisms. The mode of nutrition of, organisms in this group can be either by, synthesising their own food (autotrophic) or, getting it fr om the environment, (heterotrophic). This group includes bacteria,, blue-green algae or cyanobacteria, and, mycoplasma. Some examples are shown, in Fig. 7.1., , Resting, spore, , Bacteria, , Heterocyst, , Anabaena, , Fig. 7.1: Monera, , 7.3.2 PROTISTA, This group includes many kinds of unicellular, eukaryotic organisms. Some of these, organisms use appendages, such as hair-like, cilia or whip-like flagella for moving around., Their mode of nutrition can be autotrophic, or heterotrophic. Examples are unicellular, algae, diatoms and protozoans (see Fig. 7.2, for examples)., , L IVING O RGANISMS, , 83, , 2019-2020

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protoplasm of a host organism for food. They, are called parasites. Many of them have the, capacity to become multicellular organisms at, certain stages in their lives. They have cellwalls made of a tough complex sugar called, chitin. Examples are yeasts, molds and, mushrooms (see Fig. 7.3 for examples)., , Macronucleus, Micronucleus, , Waste, , Paramecium, , Nucleus, , Saccharomyces, (Yeast), , Penicillium, (Mold), , Agaricus, (Mushroom), , Fig. 7.3: Fungi, , Some fungal species live in permanent, mutually dependent relationships with bluegreen algae (or cyanobacteria). Such, relationships are called symbiotic. These, symbiobic life forms are called lichens. We have, all seen lichens as the slow-growing large, coloured patches on the bark of trees., , Amoeba, , Flagellum (long), , 7.3.4 PLANTAE, These are multicellular eukaryotes with cell, walls. They are autotrophs and use, chlorophyll for photosynthesis. Thus, all, plants are included in this group. Since plants, and animals are most visible forms of the, diversity of life around us, we will look at the, subgroups in this category later (section 7.4)., , Nucleus, , Euglena, Fig. 7.2: Protozoa, , 7.3.3 FUNGI, , 7.3.5 ANIMALIA, , These are heterotrophic eukaryotic, organisms. Some of them use decaying organic, material as food and are therefore called, saprotrophs. Others require a living, , These include all organisms which are, multicellular eukaryotes without cell walls., They are heterotrophs. Again, we will look at, their subgroups a little later in section 7.5., , SCIENCE, , 84, , 2019-2020

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Fig. 7.4: The Five Kingdom classification, , Q, , 7.4 Plantae, , uestions, , D IVERSITY, , 1. What is the criterion for, classification of organisms as, belonging to kingdom Monera or, Protista?, 2. In which kingdom will you place, an organism which is singlecelled,, eukaryotic, and, photosynthetic?, 3. In the hierarchy of classification,, which grouping will have the, smallest number of organisms, with, maximum, common, characteristics and which will, have the largest number of, organisms?, IN, , The first level of classification among plants, depends on whether the plant body has welldifferentiated, distinct parts. The next level of, classification is based on whether the, differentiated plant body has special tissues, for the transport of water and other, substances. Further classification looks at the, ability to bear seeds and whether the seeds, are enclosed within fruits., , 7.4.1 THALLOPHYTA, Plants that do not have well-differentiated body, design fall in this group. The plants in this, group are commonly called algae. These plants, , L IVING O RGANISMS, , 85, , 2019-2020

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are predominantly aquatic. Examples are, Spirogyra, Ulothrix, Cladophora, Ulva and, Chara (see Fig. 7.5)., , 7.4.2 BRYOPHYTA, These are called the amphibians of the plant, kingdom. The plant body is commonly, differentiated to form stem and leaf-like, structures. However, there is no specialised, tissue for the conduction of water and other, substances from one part of the plant body, to another. Examples are moss (Funaria) and, Marchantia (see Fig. 7.6)., , Cladophora, Ulothrix, , Riccia, , Cell-wall, Chloroplast, Pyrenoids, Nucleus, Cytoplasm, , Ulva, , Marchantia, , Funaria, , Spirogyra, Fig. 7.6: Some common bryophytes, , 7.4.3 PTERIDOPHYTA, , Chara, Fig. 7.5: Thallophyta – Algae, , In this group, the plant body is differentiated, into roots, stem and leaves and has, specialised tissue for the conduction of water, and other substances from one part of the, plant body to another. Some examples are, Marsilea, ferns and horse-tails (see Fig. 7.7)., The reproductive organs of plants in all, these three groups are very inconspicuous,, and they are therefore called ‘cryptogams’, or, ‘those with hidden reproductive organs’., On the other hand, plants with welldifferentiated reproductive parts that, ultimately make seeds are called, SCIENCE, , 86, , 2019-2020

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7.4.5 ANGIOSPERMS, Leaf, , Stem, Root, , Fern, , Marsilea, Fig. 7.7: Pteridophyta, , phanerogams. Seeds are the result of sexual, reproduction process. They consist of the, embryo along with stored food, which assists, for the initial growth of the embryo during, germination. This group is further classified,, based on whether the seeds are naked or, enclosed in fruits, giving us two groups:, gymnosperms and angiosperms., , This word is made from two Greek words:, angio means covered and sperma– means, seed. These are also called flowering plants., The seeds develop inside an ovary which is, modified to become a fruit. Plant embryos in, seeds have structures called cotyledons., Cotyledons are called ‘seed leaves’ because in, many instances they emerge and become, green when the seed germinates. The, angiosperms are divided into two groups on, the basis of the number of cotyledons present, in the seed. Plants with seeds having a single, cotyledon are called monocotyledonous or, monocots. Plants with seeds having two, cotyledons are called dicots (see Figs. 7.9, and 7.10)., , 7.4.4 GYMNOSPERMS, This term is derived from two Greek words:, gymno– means naked and sperma– means, seed. The plants of this group bear naked, seeds and are usually perennial, evergreen, and woody. Examples are pines and deodar, (see Fig. 7.8 for examples)., , Fig. 7.9: Monocot, , Pinus, , Cycas, , Fig. 7.8: Gymnosperms, D IVERSITY, , IN, , Fig. 7.10: Dicot, , L IVING O RGANISMS, , 87, , 2019-2020

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Fig. 7.11: Classification of plants, , Activity ______________ 7.2, , •, , Soak seeds of green gram, wheat,, maize, peas and tamarind. Once they, become tender, try to split the seed. Do, all the seeds break into two nearly, equal halves?, The seeds that do are the dicot seeds, and the seeds that don’t are the, monocot seeds., Now take a look at the roots, leaves and, flowers of these plants., Are the roots tap-roots or fibrous?, Do the leaves have parallel or reticulate, venation?, , •, , •, , •, •, , •, •, , Q, , How many petals are found in the, flower of these plants?, Can you write down further, characteristics of monocots and dicots, on the basis of these observations?, , uestions, 1. Which division among plants has, the simplest organisms?, 2. How are pteridophytes different, from the phanerogams?, 3. How do gymnosper ms and, angiosperms differ from each, other?, SCIENCE, , 88, , 2019-2020

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7.5 Animalia, These are organisms which are eukaryotic,, multicellular and heterotrophic. Their cells, do not have cell-walls. Most animals are, mobile., They are further classified based on the, extent and type of the body design, differentiation found., , layers of cells: one makes up cells on the, outside of the body, and the other makes the, inner lining of the body. Some of these species, live in colonies (corals), while others have a, solitary like–span (Hydra). Jellyfish and sea, anemones are common examples (see, Fig. 7.13)., Tentacles, , Tentacles, Stinging cell, , 7.5.1 PORIFERA, The word Porifera means organisms with, holes. These are non-motile animals attached, to some solid support. There are holes or, ‘pores’, all over the body. These lead to a canal, system that helps in circulating water, throughout the body to bring in food and, oxygen. These animals are covered with a, hard outside layer or skeleton. The body, design involves very minimal differentiation, and division into tissues. They are commonly, called sponges, and are mainly found in, marine habitats. Some examples are shown, in Fig. 7.12., , Mouth, , Epidermis, Mesoglea, Gastrodermis, Gastrovascular, cavity, , Sea anemone, Foot, , Hydra, , Fig. 7.13: Coelenterata, , 7.5.3 PLATYHELMINTHES, , Euplectella, , Sycon, , Spongilla, Fig. 7.12: Porifera, , 7.5.2 COELENTERATA (CNIDARIA), These are animals living in water. They show, more body design differentiation. There is a, cavity in the body. The body is made of two, D IVERSITY, , IN, , The body of animals in this group is far more, complexly designed than in the two other, groups we have considered so far. The body, is bilaterally symmetrical, meaning that the, left and the right halves of the body have the, same design. There are three layers of cells, from which differentiated tissues can be, made, which is why such animals are called, triploblastic. This allows outside and inside, body linings as well as some organs to be, made. There is thus some degree of tissue, formation. However, there is no true internal, body cavity or coelom, in which welldeveloped organs can be accommodated. The, body is flattened dorsoventrally (meaning from, top to bottom), which is why these animals are, called flatworms. They are either free-living or, parasitic. Some examples are free-living, animals like planarians, or parasitic animals, like liverflukes (see Fig. 7.14 for examples)., , L IVING O RGANISMS, , 89, , 2019-2020

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Eyes, , 7.5.5 ANNELIDA, , Branched, gastrovascular, cavity, , Scolex, Acetabulum, , Annelid animals are also bilaterally, symmetrical and triploblastic, but in addition, they have a true body cavity. This allows true, organs to be packaged in the body structure., There is, thus, extensive organ differentiation., This differentiation occurs in a segmental, fashion, with the segments lined up one after, the other from head to tail. These animals, are found in a variety of habitats– fresh water,, marine water as well as land. Earthworms, and leeches are familiar examples (see, Fig. 7.16)., , Sucker, Neck, , Pharynx, Mouth, and anus, , Liverfluke, , Planaria, , Tape worm, , Fig. 7.14: Platyhelminthes, Tentacle, , 7.5.4 NEMATODA, , Palp, , The nematode body is also bilaterally, symmetrical and triploblastic. However, the, body is cylindrical rather than flattened., There are tissues, but no real organs,, although a sort of body cavity or a pseudocoelom, is present. These are very familiar, as parasitic worms causing diseases, such, as the worms causing elephantiasis (filarial, worms) or the worms in the intestines, (roundworm or pinworms). Some examples, are shown in Fig. 7.15., , Parapodia, , Genital, papillae, , Anus, , Parapodia, , Nereis, , Female, , Earthworm, , Leech, , Fig. 7.16: Annelida, , 7.5.6 ARTHROPODA, Male, , Ascaris, , Wuchereria, , Fig. 7.15: Nematoda (Aschelminthes), , This is probably the largest group of animals., These animals are bilaterally symmetrical and, segmented. There is an open circulatory, system, and so the blood does not flow in welldefined blood vessels. The coelomic cavity is, blood-filled. They have jointed legs (the word, ‘arthropod’ means ‘jointed legs’). Some, familiar examples are prawns, butterflies,, houseflies, spiders, scorpions and crabs (see, Fig. 7.17)., SCIENCE, , 90, , 2019-2020

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7.5.8 ECHINODERMATA, , Aranea(Spider), Palaemon, (Prawn), , Palamnaeus, (Scorpion), , Butterfly, , In Greek, echinos means hedgehog (spiny, mammal), and derma means skin. Thus, these, are spiny skinned organisms. These are, exclusively free-living marine animals. They, are triploblastic and have a coelomic cavity., They also have a peculiar water-driven tube, system that they use for moving around. They, have hard calcium carbonate structures that, they use as a skeleton. Examples are sea-stars, and sea urchins (see Fig. 7.19)., , Pariplaneta, (Cockroach), Musca, (House fly), Scolopendra, (Centipede), , Fig. 7.17: Arthropoda, Antedon, (feather star), , 7.5.7 MOLLUSCA, , Holothuria, (sea cucumber), , In the animals of this group, there is bilateral, symmetry. The coelomic cavity is reduced., There is little segmentation. They have an, open circulatory system and kidney-like, organs for excretion. There is a foot that is, used for moving around. Examples are snails, and mussels (see Fig. 7.18)., Echinus (sea urchin), , Asterias (sea-star), , Fig. 7.19: Echinodermata, , 7.5.9 PROTOCHORDATA, , Chiton, Octopus, , Unio, Pila, , Fig. 7.18: Mollusca, D IVERSITY, , IN, , These animals are bilaterally symmetrical,, triploblastic and have a coelom. In addition,, they show a new feature of body design,, namely a notochord, at least at some stages, during their lives. The notochord is a long, rod-like support structure (chord=string) that, runs along the back of the animal separating, the nervous tissue from the gut. It provides a, place for muscles to attach for ease of, movement. Protochordates may not have a, proper notochord present at all stages in their, lives or for the entire length of the animal., Protochordates are marine animals. Examples, are Balanoglossus, Herdmania and, Amphioxus (see Fig. 7.20)., , L IVING O RGANISMS, , 91, , 2019-2020

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scaleless. They are ectoparasites or borers of, other vertebrates. Petromyzon (Lamprey) and, Myxine (Hagfish) are examples., , Proboscis, Collarette, , 7.5.10 (ii) PISCES, , Collar, , Anus, , These are fish. They are exclusively aquatic, animals. Their skin is covered with scales/, plates. They obtain oxygen dissolved in water, by using gills. The body is streamlined, and a, muscular tail is used for movement. They are, cold-blooded and their hearts have only two, chambers, unlike the four that humans have., They lay eggs. We can think of many kinds of, fish, some with skeletons made entirely of, cartilage, such as sharks, and some with a, skeleton made of both bone and cartilage, such, as tuna or rohu [see examples in Figs. 7.22 (a), and 7.22 (b)]., , Branchial region, Gill pores, , Posthepatic, region, , Dorsally, curved, genital wings, , Middosrsal, ridge, Hepatic caeca, Hepatic region, , Fig. 7.20: Protochordata: Balanoglossus, , 7.5.10 VERTEBRATA, These animals have a true vertebral column, and internal skeleton, allowing a completely, different distribution of muscle attachment, points to be used for movement., Vertebrates are bilaterally symmetrical,, triploblastic, coelomic and segmented, with, complex differentiation of body tissues and, organs. All chordates possess the following, features:, (i) have a notochord, (ii) have a dorsal nerve cord, (iii) are triploblastic, (iv) have paired gill pouches, (v) are coelomate., Vertebrates are grouped into six classes., , 7.5.10 (i) CYCLOSTOMATA, , Synchiropus splendidus, (Mandarin fish), , Caulophyryne jordani, (Angler fish), , Pterois volitans, (Lion fish), Eye, Spiracle, , Pelvic fin, Dorsal fin, Tail, , Cyclostomes are jawless vertebrates. They are, characterised by having an elongated eel-like, body, circular mouth, slimy skin and are, , Sting ray, , Caudal fin, Electric ray (Torpedo), , Dorsal fin, , Eye, , Tail, , Mouth Gills, , Pectoral Pelvic, fin, fin, Scoliodon (Dog fish), , Fig. 7.21: A jawless vertebrate: Petromyzon, , Fig. 7.22 (a): Pisces, SCIENCE, , 92, , 2019-2020

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Eye, , 7.5.10 (iv) REPTILIA, , Head, Nostril, Mouth, , Caudal, fin, , Pelvic, fin, , Pectoral Mouth, Brood, fin, pouch, , Pectoral, fin, Dorsal, fin, , Tail, Male Hippocampus, (Sea horse), , Labeo rohita (Rohu), , Wing like pectoral, , These animals are cold-blooded, have scales, and breathe through lungs. While most of, them have a three-chambered heart, crocodiles, have four heart chambers. They lay eggs with, tough coverings and do not need to lay their, eggs in water, unlike amphibians. Snakes,, turtles, lizards and crocodiles fall in this, category (see Fig. 7.24)., , Scales, Pelvic fin, , Tail, Turtle, , Exocoetus (Flying fish), , Chameleon, , Anabas (Climbing perch), , Fig. 7.22 (b): Pisces, , 7.5.10 (iii) AMPHIBIA, These animals differ from the fish in the lack, of scales, in having mucus glands in the skin,, and a three-chambered heart. Respiration is, through either gills or lungs. They lay eggs., These animals are found both in water and on, land. Frogs, toads and salamanders are some, examples (see Fig. 7.23)., , King Cobra, , House wall lizard, (Hemidactylus), Flying lizard (Draco), , Salamander, Toad, , Fig. 7.24: Reptilia, , 7.5.10 (v) AVES, , Rana tigrina, (Common frog), , Hyla (Tree frog), , Fig. 7.23: Amphibia, D IVERSITY, , IN, , These are warm-blooded animals and have a, four-chambered heart. They lay eggs. There, is an outside covering of feathers, and two, forelimbs are modified for flight. They breathe, through lungs. All birds fall in this category, (see Fig. 7.25 for examples)., , L IVING O RGANISMS, , 93, , 2019-2020

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Whale, , White Stork, (Ciconia ciconia), , Rat, Human, Bat, , Cat, Ostrich, (Struthio camelus), Male Tufted Duck, (Aythya fuligula), , Fig. 7.26: Mammalia, , Q, , Sparrow, Pigeon, , Crow, , Fig. 7.25: Aves (birds), , 7.5.10 (vi) MAMMALIA, Mammals are warm-blooded animals with, four-chambered hearts. They have mammary, glands for the production of milk to nourish, their young. Their skin has hairs as well as, sweat and oil glands. Most mammals familiar, to us produce live young ones. However, a, few of them, like the platypus and the echidna, lay eggs, and some, like kangaroos give birth, to very poorly developed young ones. Some, examples are shown in Fig. 7.26., The scheme of classification of animals is, shown in Fig. 7.27., , uestions, 1. How do poriferan animals differ, from coelenterate animals?, 2. How do annelid animals differ, from arthropods?, 3. What are the differences between, amphibians and reptiles?, 4. What are the differences between, animals belonging to the Aves, group and those in the mammalia, group?, , Carolus Linnaeus (Karl, von Linne) was born in, Sweden and was a doctor, by professsion. He was, interested in the study of, plants. At the age of 22, he, published his first paper on, plants. While serving as a, personal physician of a Carolus Linnaeus, (1707-1778), wealthy, government, official, he studied the diversity of plants, in his employer’s garden. Later, he, published 14 papers and also brought out, the famous book Systema Naturae from, which all fundamental taxonomical, researches have taken off. His system of, classification was a simple scheme for, arranging plants so as to be able to identify, them again., SCIENCE, , 94, , 2019-2020

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Fig. 7.27: Classification of animals, D IVERSITY, , IN, , L IVING O RGANISMS, , 95, , 2019-2020

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7.6 Nomenclature, Why is there a need for systematic naming of, living organisms?, , Activity ______________ 7.3, Find out the names of the following, animals and plants in as many, languages as you can:, 1. Tiger, 2. Peacock, 3. Ant, 4. Neem, 5. Lotus, 6. Potato, , •, , As you might be able to appreciate, it, would be difficult for people speaking or, writing in different languages to know when, they are talking about the same organism. This, problem was resolved by agreeing upon a, ‘scientific’ name for organisms in the same, manner that chemical symbols and formulae, for various substances are used the world over., The scientific name for an organism is thus, unique and can be used to identify it, anywhere in the world., The system of scientific naming or, nomenclature we use today was introduced, by Carolus Linnaeus in the eighteenth, century. The scientific name of an organism is, , the result of the process of classification, which puts it along with the organisms it is, most related to. But when we actually name, the species, we do not list out the whole, hierarchy of groups it belongs to. Instead, we, limit ourselves to writing the name of the genus, and species of that particular organism. World, over, it has been agreed that both these names, will be used in Latin forms., Certain conventions are followed while, writing the scientific names:, 1. The name of the genus begins with a, capital letter., 2. The name of the species begins with a, small letter., 3. When printed, the scientific name is, given in italics., 4. When written by hand, the genus name, and the species name have to be, underlined separately., , Activity ______________ 7.4, •, , Find out the scientific names of any, five common animals and plants. Do, these names have anything in, common with the names you normally, use to identify them?, , What, you have, learnt, •, •, , •, , •, , Classification helps us in exploring the diversity of life forms., The major characteristics considered for classifying all, organisms into five major kingdoms are:, (a) whether they are made of prokaryotic or eukaryotic cells, (b) whether the cells are living singly or organised into, multi-cellular and thus complex organisms, (c) whether the cells have a cell-wall and whether they, prepare their own food., All living organisms are divided on the above bases into five, kingdoms, namely Monera, Protista, Fungi, Plantae and, Animalia., The classification of life forms is related to their evolution., SCIENCE, , 96, , 2019-2020

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•, •, •, , •, •, , Plantae and Animalia are further divided into subdivisions, on the basis of increasing complexity of body organisation., Plants are divided into five groups: Thallophytes, Bryophytes,, Pteridophytes, Gymnosperms and Angiosperms., Animals are divided into ten groups: Porifera, Coelenterata,, Platyhelminthes, Nematoda, Annelida, Arthropoda, Mollusca,, Echinodermata, Protochordata and Vertebrata., The binomial nomenclature makes for a uniform way of, identification of the vast diversity of life around us., The binomial nomenclature is made up of two words – a generic, name and a specific name., , Exercises, 1., 2., 3., 4., 5., 6., , D IVERSITY, , IN, , What are the advantages of classifying organisms?, How would you choose between two characteristics to be used, for developing a hierarchy in classification?, Explain the basis for grouping organisms into five kingdoms., What are the major divisions in the Plantae? What is the basis, for these divisions?, How are the criteria for deciding divisions in plants different, from the criteria for deciding the subgroups among animals?, Explain how animals in Vertebrata are classified into further, subgroups., , L IVING O RGANISMS, , 97, , 2019-2020

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Chapter 8, MOTION, In everyday life, we see some objects at rest, and others in motion. Birds fly, fish swim,, blood flows through veins and arteries, and, cars move. Atoms, molecules, planets, stars, and galaxies are all in motion. We often, perceive an object to be in motion when its, position changes with time. However, there, are situations where the motion is inferred, through indirect evidences. For example, we, infer the motion of air by observing the, movement of dust and the movement of leaves, and branches of trees. What causes the, phenomena of sunrise, sunset and changing, of seasons? Is it due to the motion of the, earth? If it is true, why don’t we directly, perceive the motion of the earth?, An object may appear to be moving for, one person and stationary for some other. For, the passengers in a moving bus, the roadside, trees appear to be moving backwards. A, person standing on the road–side perceives, the bus alongwith the passengers as moving., However, a passenger inside the bus sees his, fellow passengers to be at rest. What do these, observations indicate?, Most motions are complex. Some objects, may move in a straight line, others may take, a circular path. Some may rotate and a few, others may vibrate. There may be situations, involving a combination of these. In this, chapter, we shall first learn to describe the, motion of objects along a straight line. We, shall also learn to express such motions, through simple equations and graphs. Later,, we shall discuss ways of describing circular, motion., , Activity ______________ 8.1, •, , Discuss whether the walls of your, classroom are at rest or in motion., , Activity ______________ 8.2, •, , •, , Have you ever experienced that the, train in which you are sitting appears, to move while it is at rest?, Discuss and share your experience., , Think and Act, We sometimes are endangered by the, motion of objects around us, especially, if that motion is erratic and, uncontrolled as observed in a flooded, river, a hurricane or a tsunami. On the, other hand, controlled motion can be a, service to human beings such as in the, generation of hydro-electric power. Do, you feel the necessity to study the, erratic motion of some objects and, learn to control them?, , 8.1 Describing Motion, We describe the location of an object by, specifying a reference point. Let us, understand this by an example. Let us, assume that a school in a village is 2 km north, of the railway station. We have specified the, position of the school with respect to the, railway station. In this example, the railway, station is the reference point. We could have, also chosen other reference points according, to our convenience. Therefore, to describe the, position of an object we need to specify a, reference point called the origin., , 2019-2020

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8.1.1 MOTION ALONG A STRAIGHT LINE, The simplest type of motion is the motion, along a straight line. We shall first learn to, describe this by an example. Consider the, motion of an object moving along a straight, path. The object starts its journey from O, which is treated as its reference point, (Fig. 8.1). Let A, B and C represent the, position of the object at different instants. At, first, the object moves through C and B and, reaches A. Then it moves back along the same, path and reaches C through B., , while the magnitude of displacement = 35 km., Thus, the magnitude of displacement (35 km), is not equal to the path length (85 km)., Further, we will notice that the magnitude of, the displacement for a course of motion may, be zero but the corresponding distance, covered is not zero. If we consider the object, to travel back to O, the final position concides, with the initial position, and therefore, the, displacement is zero. However, the distance, covered in this journey is OA + AO = 60 km +, 60 km = 120 km. Thus, two different physical, quantities — the distance and the, , Fig. 8.1: Positions of an object on a straight line path, , The total path length covered by the object, is OA + AC, that is 60 km + 35 km = 95 km., This is the distance covered by the object. To, describe distance we need to specify only the, numerical value and not the direction of, motion. There are certain quantities which, are described by specifying only their, numerical values. The numerical value of a, physical quantity is its magnitude. From this, example, can you find out the distance of the, final position C of the object from the initial, position O? This difference will give you the, numerical value of the displacement of the, object from O to C through A. The shortest, distance measured from the initial to the final, position of an object is known as the, displacement., Can the magnitude of the displacement, be equal to the distance travelled by an, object? Consider the example given in, (Fig. 8.1). For motion of the object from O to, A, the distance covered is 60 km and the, magnitude of displacement is also 60 km., During its motion from O to A and back to B,, the distance covered = 60 km + 25 km = 85 km, , displacement, are used to describe the overall, motion of an object and to locate its final, position with reference to its initial position, at a given time., , Activity ______________ 8.3, •, •, , •, •, , Take a metre scale and a long rope., Walk from one corner of a basket-ball, court to its oppposite corner along its, sides., Measure the distance covered by you, and magnitude of the displacement., What dif ference would you notice, between the two in this case?, , Activity ______________ 8.4, •, , •, , Automobiles are fitted with a device, that shows the distance travelled. Such, a device is known as an odometer. A, car is driven from Bhubaneshwar to, New Delhi. The difference between the, final reading and the initial reading of, the odometer is 1850 km., Find the magnitude of the displacement, between Bhubaneshwar and New Delhi, by using the Road Map of India., 99, , MOTION, , 2019-2020

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Q, , uestions, , Table 8.1, , 1. An object has moved through a, distance. Can it have zero, displacement? If yes, support, your answer with an example., 2. A farmer moves along the, boundary of a square field of side, 10 m in 40 s. What will be the, magnitude of displacement of the, farmer at the end of 2 minutes 20, seconds from his initial position?, 3. Which of the following is true for, displacement?, (a) It cannot be zero., (b) Its magnitude is greater than, the distance travelled by the, object., , 8.1.2 U NIFORM, , Time, , 9:30 am, 9:45 am, 10:00 am, 10:15 am, 10:30 am, 10:45 am, 11:00 am, , Distance, travelled by, object A in m, , Distance, travelled by, object B in m, , 10, 20, 30, 40, 50, 60, 70, , 12, 19, 23, 35, 37, 41, 44, , 8.2 Measuring the Rate of Motion, , MOTION AND N O N -, , UNIFORM MOTION, , Consider an object moving along a straight, line. Let it travel 5 m in the first second,, 5 m more in the next second, 5 m in the, third second and 5 m in the fourth second., In this case, the object covers 5 m in each, second. As the object covers equal distances, in equal intervals of time, it is said to be in, uniform motion. The time interval in this, motion should be small. In our day-to-day, life, we come across motions where objects, cover unequal distances in equal intervals, of time, for example, when a car is moving, on a crowded street or a person is jogging, in a park. These are some instances of, non-uniform motion., , (a), , Activity ______________ 8.5, •, , •, , The data regarding the motion of two, different objects A and B are given in, Table 8.1., Examine them carefully and state, whether the motion of the objects is, uniform or non-uniform., , (b), Fig. 8.2, , 100, , SCIENCE, , 2019-2020

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Look at the situations given in Fig. 8.2. If, the bowling speed is 143 km h–1 in Fig. 8.2(a), what does it mean? What do you understand, from the signboard in Fig. 8.2(b)?, Different objects may take different, amounts of time to cover a given distance., Some of them move fast and some move, slowly. The rate at which objects move can, be different. Also, different objects can move, at the same rate. One of the ways of, measuring the rate of motion of an object is, to find out the distance travelled by the object, in unit time. This quantity is referred to as, speed. The SI unit of speed is metre per, second. This is represented by the symbol, m s–1 or m/s. The other units of speed include, centimetre per second (cm s–1) and kilometre, per hour (km h–1). To specify the speed of an, object, we require only its magnitude. The, speed of an object need not be constant. In, most cases, objects will be in non-uniform, motion. Therefore, we describe the rate of, motion of such objects in terms of their, average speed. The average speed of an object, is obtained by dividing the total distance, travelled by the total time taken. That is,, average speed =, , Total distance travelled, Total time taken, , If an object travels a distance s in time t then, its speed v is,, v=, , s, t, , (8.1), , Let us understand this by an example. A, car travels a distance of 100 km in 2 h. Its, average speed is 50 km h–1. The car might, not have travelled at 50 km h–1 all the time., Sometimes it might have travelled faster and, sometimes slower than this., , Average speed =, , =, , Total distance travelled, Total time taken, 32 m, –1, 6 s = 5.33 m s, , Therefore, the average speed of the object, is 5.33 m s–1., , 8.2.1 SPEED WITH DIRECTION, The rate of motion of an object can be more, comprehensive if we specify its direction of, motion along with its speed. The quantity that, specifies both these aspects is called velocity., Velocity is the speed of an object moving in a, definite direction. The velocity of an object, can be uniform or variable. It can be changed, by changing the object’s speed, direction of, motion or both. When an object is moving, along a straight line at a variable speed, we, can express the magnitude of its rate of, motion in terms of average velocity. It is, calculated in the same way as we calculate, average speed., In case the velocity of the object is, changing at a uniform rate, then average, velocity is given by the arithmetic mean of, initial velocity and final velocity for a given, period of time. That is,, average velocity =, , initial velocity + final velocity, 2, , Mathematically, vav =, , u+v, 2, , (8.2), , where vav is the average velocity, u is the initial, velocity and v is the final velocity of the object., Speed and velocity have the same units,, that is, m s–1 or m/s., , Example 8.1 An object travels 16 m in 4 s, and then another 16 m in 2 s. What is, the average speed of the object?, Solution:, Total distance travelled by the object =, 16 m + 16 m = 32 m, Total time taken = 4 s + 2 s = 6 s, , Activity ______________ 8.6, •, , Measure the time it takes you to walk, from your house to your bus stop or, the school. If you consider that your, average walking speed is 4 km h–1,, estimate the distance of the bus stop, or school from your house., 101, , MOTION, , 2019-2020

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Activity ______________ 8.7, •, , At a time when it is cloudy, there may, be frequent thunder and lightning. The, sound of thunder takes some time to, reach you after you see the lightning., Can you answer why this happens?, Measure this time interval using a, digital wrist watch or a stop watch., Calculate the distance of the nearest, point of lightning. (Speed of sound in, air = 346 m s-1.), , •, •, •, , Q, , uestions, 1. Distinguish between speed and, velocity., 2. Under what condition(s) is the, magnitude of average velocity of, an object equal to its average, speed?, 3. What does the odometer of an, automobile measure?, 4. What does the path of an object, look like when it is in uniform, motion?, 5. During an experiment, a signal, from a spaceship reached the, ground station in five minutes., What was the distance of the, spaceship from the ground, station? The signal travels at the, speed of light, that is, 3 × 108, m s–1., , Example 8.2 The odometer of a car reads, 2000 km at the start of a trip and, 2400 km at the end of the trip. If the, trip took 8 h, calculate the average, speed of the car in km h–1 and m s–1., Solution:, Distance covered by the car,, s = 2400 km – 2000 km = 400 km, Time elapsed, t = 8 h, Average speed of the car is,, vav =, , s, t, , =, , 400 km, 8h, , = 50 km h–1, , = 50, , km, , ×, , 1000 m, , h, , 1h, , ×, , 1km, , 3600s, , –1, , = 13.9 m s, The average speed of the car is, 50 km h–1 or 13.9 m s–1., , Example 8.3 Usha swims in a 90 m long, pool. She covers 180 m in one minute, by swimming from one end to the other, and back along the same straight path., Find the average speed and average, velocity of Usha., Solution:, Total distance covered by Usha in 1 min, is 180 m., Displacement of Usha in 1 min = 0 m, Total distance covered, , Average speed =, , Total time taken, 180 m, , =, , 1min, , =, , 180 m, 1min, , ×, , 1 min, 60 s, , -1, , =3ms, , Displacement, , Average velocity =, , Total time taken, , 0m, =, , 60 s, , = 0 m s–1, , The average speed of Usha is 3 m s–1, and her average velocity is 0 m s–1., , 8.3 Rate of Change of Velocity, During uniform motion of an object along a, straight line, the velocity remains constant, with time. In this case, the change in velocity, of the object for any time interval is zero., However, in non-uniform motion, velocity, varies with time. It has different values at, different instants and at different points of, the path. Thus, the change in velocity of the, object during any time interval is not zero., Can we now express the change in velocity of, an object?, , 102, , SCIENCE, , 2019-2020

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attain a velocity of 6 m s–1 in 30 s. Then, he applies brakes such that the velocity, of the bicycle comes down to 4 m s-1 in, the next 5 s. Calculate the acceleration, of the bicycle in both the cases., , To answer such a question, we have to, introduce another physical quantity called, acceleration, which is a measure of the, change in the velocity of an object per unit, time. That is,, , Solution:, , change in velocity, acceleration =, , time taken, , In the first case:, initial velocity, u = 0 ;, final velocity, v = 6 m s–1 ;, time, t = 30 s ., From Eq. (8.3), we have, , If the velocity of an object changes from, an initial value u to the final value v in time t,, the acceleration a is,, a=, , v–u, t, , (8.3), , v–u, , a=, , t, , This kind of motion is known as, accelerated motion. The acceleration is taken, to be positive if it is in the direction of velocity, and negative when it is opposite to the, direction of velocity. The SI unit of, acceleration is m s–2 ., If an object travels in a straight line and, its velocity increases or decreases by equal, amounts in equal intervals of time, then the, acceleration of the object is said to be, uniform. The motion of a freely falling body, is an example of uniformly accelerated, motion. On the other hand, an object can, travel with non-uniform acceleration if its, velocity changes at a non-uniform rate. For, example, if a car travelling along a straight, road increases its speed by unequal amounts, in equal intervals of time, then the car is said, to be moving with non-uniform acceleration., , Substituting the given values of u,v and, t in the above equation, we get, , (6 m s, a=, , –1, , – 0m s, , –1, , ), , 30 s, , = 0.2 m s–2, , In the second case:, initial velocity, u = 6 m s–1;, final velocity, v = 4 m s–1;, time, t = 5 s., , (4 m s, Then, a =, , –1, , – 6m s, , –1, , ), , 5s, , = –0.4 m s–2 ., , The acceleration of the bicycle in the, first case is 0.2 m s–2 and in the second, case, it is –0.4 m s–2., , Activity ______________ 8.8, •, , •, , In your everyday life you come across, a range of motions in which, (a) acceleration is in the direction of, motion,, (b) acceleration is against the, direction of motion,, (c) acceleration is uniform,, (d) acceleration is non-uniform., Can you identify one example each, for the above type of motion?, , Q, , Example 8.4 Starting from a stationary, position, Rahul paddles his bicycle to, , uestions, 1. When will you say a body is in, (i) uniform acceleration? (ii) nonuniform acceleration?, 2. A bus decreases its speed from, 80 km h–1 to 60 km h–1 in 5 s., Find the acceleration of the bus., 3. A train starting from a railway, station and moving with uniform, acceleration attains a speed, 40 km h–1 in 10 minutes. Find its, acceleration., 103, , MOTION, , 2019-2020

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8.4 Graphical Representation of, Motion, Graphs provide a convenient method to, present basic information about a variety of, events. For example, in the telecast of a, one-day cricket match, vertical bar graphs, show the run rate of a team in each over. As, you have studied in mathematics, a straight, line graph helps in solving a linear equation, having two variables., To describe the motion of an object, we, can use line graphs. In this case, line graphs, show dependence of one physical quantity,, such as distance or velocity, on another, quantity, such as time., , 8.4.1 DISTANCE–TIME GRAPHS, The change in the position of an object with, time can be represented on the distance-time, graph adopting a convenient scale of choice., In this graph, time is taken along the x–axis, and distance is taken along the y-axis., Distance-time graphs can be employed under, various conditions where objects move with, uniform speed, non-uniform speed, remain, at rest etc., , distance travelled by the object is directly, proportional to time taken. Thus, for uniform, speed, a graph of distance travelled against, time is a straight line, as shown in Fig. 8.3., The portion OB of the graph shows that the, distance is increasing at a uniform rate. Note, that, you can also use the term uniform, velocity in place of uniform speed if you take, the magnitude of displacement equal to the, distance travelled by the object along the, y-axis., We can use the distance-time graph to, determine the speed of an object. To do so,, consider a small part AB of the distance-time, graph shown in Fig 8.3. Draw a line parallel, to the x-axis from point A and another line, parallel to the y-axis from point B. These two, lines meet each other at point C to form a, triangle ABC. Now, on the graph, AC denotes, the time interval (t2 – t1) while BC corresponds, to the distance (s2 – s1). We can see from the, graph that as the object moves from the point, A to B, it covers a distance (s2 – s1) in time, (t2 – t1). The speed, v of the object, therefore, can be represented as, v=, , s2 – s1, , (8.4), , t 2 – t1, , We can also plot the distance-time graph, for accelerated motion. Table 8.2 shows the, distance travelled by a car in a time interval, of two seconds., , Table 8.2: Distance travelled by a, car at regular time intervals, Time in seconds, , Distance in metres, , 0, , 0, , 2, , 1, , 4, , 4, , 6, , 9, , 8, , 16, , 10, , 25, , 12, , 36, , Fig. 8.3: Distance-time graph of an object moving, with uniform speed, , We know that when an object travels equal, distances in equal intervals of time, it moves, with uniform speed. This shows that the, 104, , SCIENCE, , 2019-2020

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Fig. 8.4: Distance-time graph for a car moving with, non-uniform speed, , The distance-time graph for the motion, of the car is shown in Fig. 8.4. Note that the, shape of this graph is different from the earlier, distance-time graph (Fig. 8.3) for uniform, motion. The nature of this graph shows nonlinear variation of the distance travelled by, the car with time. Thus, the graph shown in, Fig 8.4 represents motion with non-uniform, speed., , 8.4.2 VELOCITY-TIME GRAPHS, The variation in velocity with time for an, object moving in a straight line can be, represented by a velocity-time graph. In this, graph, time is represented along the x-axis, , and the velocity is represented along the, y-axis. If the object moves at uniform velocity,, the height of its velocity-time graph will not, change with time (Fig. 8.5). It will be a straight, line parallel to the x-axis. Fig. 8.5 shows the, velocity-time graph for a car moving with, uniform velocity of 40 km h–1., We know that the product of velocity and, time give displacement of an object moving, with uniform velocity. The area enclosed by, velocity-time graph and the time axis will be, equal to the magnitude of the displacement., To know the distance moved by the car, between time t1 and t2 using Fig. 8.5, draw, perpendiculars from the points corresponding, to the time t1 and t2 on the graph. The velocity, of 40 km h–1 is represented by the height AC, or BD and the time (t2 – t1) is represented by, the length AB., So, the distance s moved by the car in, time (t2 – t1) can be expressed as, s = AC × CD, = [(40 km h–1) × (t2 – t1) h], = 40 (t2– t1) km, = area of the rectangle ABDC (shaded, in Fig. 8.5)., We can also study about unifor mly, accelerated motion by plotting its velocity–, time graph. Consider a car being driven along, a straight road for testing its engine. Suppose, a person sitting next to the driver records its, velocity after every 5 seconds by noting the, reading of the speedometer of the car. The, velocity of the car, in km h–1 as well as in, m s–1, at different instants of time is shown, in table 8.3., , Table 8.3: Velocity of a car at, regular instants of time, Time, (s), , Fig. 8.5: Velocity-time graph for uniform motion of, a car, , 0, 5, 10, 15, 20, 25, 30, , Velocity of the car, –1, (m s–1), (km h ), 0, 2.5, 5.0, 7.5, 10.0, 12.5, 15.0, , 0, 9, 18, 27, 36, 45, 54, 105, , MOTION, , 2019-2020

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–1, , Velocity (km h ), , In this case, the velocity-time graph for the, motion of the car is shown in Fig. 8.6. The, nature of the graph shows that velocity, changes by equal amounts in equal intervals, of time. Thus, for all uniformly accelerated, motion, the velocity-time graph is a, straight line., , Fig. 8.7: Velocity-time graphs of an object in nonuniformly accelerated motion., , Fig. 8.6: Velocity-time graph for a car moving with, uniform accelerations., , You can also determine the distance, moved by the car from its velocity-time graph., The area under the velocity-time graph gives, the distance (magnitude of displacement), moved by the car in a given interval of time., If the car would have been moving with, uniform velocity, the distance travelled by it, would be represented by the area ABCD, under the graph (Fig. 8.6). Since the, magnitude of the velocity of the car is, changing due to acceleration, the distance s, travelled by the car will be given by the area, ABCDE under the velocity-time graph, (Fig. 8.6)., That is,, s = area ABCDE, = area of the rectangle ABCD + area of, the triangle ADE, = AB × BC +, , 1, (AD × DE), 2, , In the case of non-uniformly accelerated, motion, velocity-time graphs can have any, shape., , Fig. 8.7(a) shows a velocity-time graph, that represents the motion of an object whose, velocity is decreasing with time while, Fig. 8.7 (b) shows the velocity-time graph, representing the non-uniform variation of, velocity of the object with time. Try to interpret, these graphs., , Activity ______________ 8.9, •, , The times of arrival and departure of a, train at three stations A, B and C and, the distance of stations B and C from, station A are given in table 8.4., , Table 8.4: Distances of stations B, and C from A and times of arrival, and departure of the train, Station, , A, B, C, •, , 106, , Distance, from A, (km), 0, 120, 180, , Time of Time of, arrival departure, (hours), (hours), 08:00, 11:15, 13:00, , 08:15, 11:30, 13:15, , Plot and interpret the distance-time, graph for the train assuming that its, motion between any two stations is, uniform., SCIENCE, , 2019-2020

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Activity _____________ 8.10, •, , Feroz and his sister Sania go to school, on their bicycles. Both of them start at, the same time from their home but take, different times to reach the school, although they follow the same route., Table 8.5 shows the distance travelled, by them in different times, , Table 8.5: Distance covered by Feroz, and Sania at different times on, their bicycles, Time, , Distance, travelled, by Feroz, (km), , Distance, travelled, by Sania, (km), , 8:00 am, , 0, , 0, , 8:05 am, , 1.0, , 0.8, , 8:10 am, , 1.9, , 1.6, , 8:15 am, , 2.8, , 2.3, , 8:20 am, , 3.6, , 3.0, , 8:25 am, , –, , 3.6, , •, , Q, , 8.5 Equations of Motion by, Graphical Method, When an object moves along a straight line, with uniform acceleration, it is possible to, relate its velocity, acceleration during motion, and the distance covered by it in a certain, time interval by a set of equations known as, the equations of motion. For convenience, a, set of three such equations are given below:, v = u + at, (8.5), s = ut + ½ at2, (8.6), 2 a s = v2 – u2, (8.7), where u is the initial velocity of the object, which moves with uniform acceleration a for, time t, v is the final velocity, and s is the, distance travelled by the object in time t., Eq. (8.5) describes the velocity-time relation, and Eq. (8.6) represents the position-time, relation. Eq. (8.7), which represents the, relation between the position and the velocity,, can be obtained from Eqs. (8.5) and (8.6) by, eliminating t. These three equations can be, derived by graphical method., , 8.5.1 E QUATION, , FOR VELOCITY - TIME, , RELATION, , Plot the distance-time graph for their, motions on the same scale and, interpret., , Consider the velocity-time graph of an object, that moves under uniform acceleration as, , uestions, 1. What is the nature of the, distance-time graphs for uniform, and non-uniform motion of an, object?, 2. What can you say about the, motion of an object whose, distance-time graph is a straight, line parallel to the time axis?, 3. What can you say about the, motion of an object if its speedtime graph is a straight line, parallel to the time axis?, 4. What is the quantity which is, measured by the area occupied, below the velocity-time graph?, , Fig. 8.8: Velocity-time graph to obtain the equations, of motion, 107, , MOTION, , 2019-2020

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shown in Fig. 8.8 (similar to Fig. 8.6, but now, with u ≠ 0). From this graph, you can see that, initial velocity of the object is u (at point A), and then it increases to v (at point B) in time, t. The velocity changes at a uniform rate a., In Fig. 8.8, the perpendicular lines BC and, BE are drawn from point B on the time and, the velocity axes respectively, so that the, initial velocity is represented by OA, the final, velocity is represented by BC and the time, interval t is represented by OC. BD = BC –, CD, represents the change in velocity in time, interval t., Let us draw AD parallel to OC. From the, graph, we observe that, BC = BD + DC = BD + OA, Substituting BC = v and OA = u,, we get, v, = BD + u, or, BD = v – u, (8.8), From the velocity-time graph (Fig. 8.8),, the acceleration of the object is given by, , = OA × OC +, , s=u×t+, or, , From the velocity-time graph shown in, Fig. 8.8, the distance s travelled by the object, in time t, moving under uniform acceleration, a is given by the area enclosed within the, trapezium OABC under the graph. That is,, s = area of the trapezium OABC, =, , (OA + BC) ×OC, 2, , Substituting OA = u, BC = v and OC = t,, we get, s=, , BD, t, BD = at, , (u + v ) t, , (8.11), , 2, , a=, , (8.9), , From the velocity-time relation (Eq. 8.6),, we get, , Using Eqs. (8.8) and (8.9) we get, v = u + at, , 8.5.2 E QUATION, RELATION, , 1, at2, 2, , s=ut+, , RELATION, , Substituting OC = t, we get, , or, , 1, (t ×at ), 2, , 8.5.3 EQUATION FOR POSITION–VELOCITY, , time taken, , BD, BD, =, =, AD, OC, , (8.10), , Substituting OA = u, OC = AD = t and BD = at,, we get, , Change in velocity, , a=, , 1, (AD × BD), 2, , t=, , FOR POSITION - TIME, , Let us consider that the object has travelled, a distance s in time t under uniform, acceleration a. In Fig. 8.8, the distance, travelled by the object is obtained by the area, enclosed within OABC under the velocity-time, graph AB., Thus, the distance s travelled by the object, is given by, s = area OABC (which is a trapezium), = area of the rectangle OADC + area of, the triangle ABD, , (v – u ), , (8.12), , a, , Using Eqs. (8.11) and (8.12) we have, s=, , or, , (v + u ) × (v - u ), 2a, , 2 a s = v2 – u2, , Example 8.5 A train starting from rest, attains a velocity of 72 km h –1 in, 5 minutes. Assuming that the, acceleration is uniform, find (i) the, acceleration and (ii) the distance, travelled by the train for attaining this, velocity., , 108, , SCIENCE, , 2019-2020

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Solution:, , (ii) From Eq. (8.6) we have, , We have been given, u = 0 ; v = 72 km h–1 = 20 m s-1 and, t = 5 minutes = 300 s., (i) From Eq. (8.5) we know that, , s=ut+, , 1, × 1 m s–2 × (5 s)2, 2, = 25 m + 12.5 m, , = 5 m s–1 × 5 s +, , (v – u ), , a=, , t, , = 37.5 m, –2, The acceleration of the car is 1 m s, and the distance covered is 37.5 m., , 20 m s –1 – 0 m s –1, =, 300 s, =, , 1, m s –2, 15, , Example 8.7 The brakes applied to a car, produce an acceleration of 6 m s-2 in, the opposite direction to the motion. If, the car takes 2 s to stop after the, application of brakes, calculate the, distance it travels during this time., , (ii) From Eq. (8.7) we have, 2 a s = v2 – u2 = v2 – 0, Thus,, s=, , v, , 2, , 2a, , Solution:, –1 2, , =, , We have been given, a = – 6 m s–2 ; t = 2 s and v = 0 m s–1., From Eq. (8.5) we know that, v = u + at, 0 = u + (– 6 m s–2) × 2 s, or u = 12 m s–1 ., From Eq. (8.6) we get, , (20 m s ), , 2×(1/15) m s, , –2, , = 3000 m, = 3 km, , The acceleration of the train is, , 1, m s– 2, 15, , and the distance travelled is 3 km., , s=ut+, , Example 8.6 A car accelerates uniformly, –1, –1, from 18 km h to 36 km h in 5 s., Calculate (i) the acceleration and (ii) the, distance covered by the car in that time., , v–u, a=, t, 10 m s -1 – 5 m s-1, 5s, –2, , =1ms, , 1, (–6 m s–2 ) (2 s)2, 2, , = 24 m – 12 m, = 12 m, , Thus, the car will move 12 m before it, stops after the application of brakes., Can you now appreciate why drivers, are cautioned to maintain some, distance between vehicles while, travelling on the road?, , We are given that, u = 18 km h–1 = 5 m s–1, v = 36 km h–1 = 10 m s–1 and, t = 5s., (i) From Eq. (8.5) we have, , 1, at2, 2, , = (12 m s–1 ) × (2 s) +, , Solution:, , =, , 1, at 2, 2, , Q, , uestions, 1. A bus starting from rest moves, with a uniform acceleration of, 0.1 m s-2 for 2 minutes. Find (a), the speed acquired, (b) the, distance travelled., 109, , MOTION, , 2019-2020

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2. A train is travelling at a speed, of 90 km h–1. Brakes are applied, so as to produce a unifor m, acceleration of – 0.5 m s-2. Find, how far the train will go before it, is brought to rest., 3. A trolley, while going down an, inclined, plane,, has, an, acceleration of 2 cm s-2. What will, be its velocity 3 s after the start?, 4. A racing car has a unifor m, acceleration of 4 m s -2. What, distance will it cover in 10 s after, start?, 5. A stone is thrown in a vertically, upward direction with a velocity, of 5 m s-1. If the acceleration of the, stone during its motion is 10 m s–2, in the downward direction, what, will be the height attained by the, stone and how much time will it, take to reach there?, , 8.6 Uniform Circular Motion, When the velocity of an object changes, we, say that the object is accelerating. The change, in the velocity could be due to change in its, magnitude or the direction of the motion or, both. Can you think of an example when an, object does not change its magnitude of, velocity but only its direction of motion?, , (a) Rectangular track, , (b) Hexagonal track, , Let us consider an example of the motion, of a body along a closed path. Fig 8.9 (a) shows, the path of an athlete along a rectangular, track ABCD. Let us assume that the athlete, runs at a uniform speed on the straight parts, AB, BC, CD and DA of the track. In order to, keep himself on track, he quickly changes, his speed at the corners. How many times, will the athlete have to change his direction, of motion, while he completes one round? It, is clear that to move in a rectangular track, once, he has to change his direction of motion, four times., Now, suppose instead of a rectangular, track, the athlete is running along a, hexagonal shaped path ABCDEF, as shown, in Fig. 8.9(b). In this situation, the athlete, will have to change his direction six times, while he completes one round. What if the, track was not a hexagon but a regular, octagon, with eight equal sides as shown by, ABCDEFGH in Fig. 8.9(c)? It is observed that, as the number of sides of the track increases, the athelete has to take turns more and more, often. What would happen to the shape of, the track as we go on increasing the number, of sides indefinitely? If you do this you will, notice that the shape of the track approaches, the shape of a circle and the length of each of, the sides will decrease to a point. If the athlete, moves with a velocity of constant magnitude, along the circular path, the only change in, his velocity is due to the change in the, direction of motion. The motion of the athlete, moving along a circular path is, therefore, an, example of an accelerated motion., We know that the circumference of a circle, of radius r is given by 2 πr . If the athlete takes, t seconds to go once around the circular path, of radius r, the speed v is given by, v=, , (c) Octagonal shaped track, , (d) A circular track, , Fig. 8.9: The motion of an athlete along closed tracks, of different shapes., , 2 πr, t, , (8.13), , When an object moves in a circular path, with uniform speed, its motion is called, uniform circular motion., , 110, , SCIENCE, , 2019-2020

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Activity _____________ 8.11, •, , Take a piece of thread and tie a small, piece of stone at one of its ends. Move, the stone to describe a circular path, with constant speed by holding the, thread at the other end, as shown in, Fig. 8.10., , Fig. 8.10: A stone describing a circular path with, a velocity of constant magnitude., , •, •, •, , Now, let the stone go by releasing the, thread., Can you tell the direction in which the, stone moves after it is released?, By repeating the activity for a few times, and releasing the stone at different, positions of the circular path, check, whether the direction in which the, stone moves remains the same or not., , If you carefully note, on being released, the stone moves along a straight line, tangential to the circular path. This is, because once the stone is released, it, continues to move along the direction it has, been moving at that instant. This shows that, the direction of motion changed at every point, when the stone was moving along the circular, path., When an athlete throws a hammer or a, discus in a sports meet, he/she holds the, hammer or the discus in his/her hand and, gives it a circular motion by rotating his/her, own body. Once released in the desired, direction, the hammer or discus moves in the, direction in which it was moving at the time, it was released, just like the piece of stone in, the activity described above. There are many, more familiar examples of objects moving, under uniform circular motion, such as the, motion of the moon and the earth, a satellite, in a circular orbit around the earth, a cyclist, on a circular track at constant speed, and so on., , What, you have, learnt, •, •, •, •, •, •, , Motion is a change of position; it can be described in terms of, the distance moved or the displacement., The motion of an object could be uniform or non-uniform, depending on whether its velocity is constant or changing., The speed of an object is the distance covered per unit time,, and velocity is the displacement per unit time., The acceleration of an object is the change in velocity per unit, time., Uniform and non-uniform motions of objects can be shown, through graphs., The motion of an object moving at uniform acceleration can be, described with the help of the following equations, namely, v = u + at, s = ut + ½ at2, 2as = v2 – u2, 111, , MOTION, , 2019-2020

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•, , where u is initial velocity of the object, which moves with, uniform acceleration a for time t, v is its final velocity and s is, the distance it travelled in time t., If an object moves in a circular path with uniform speed, its, motion is called uniform circular motion., , Exercises, 1. An athlete completes one round of a circular track of diameter, 200 m in 40 s. What will be the distance covered and the, displacement at the end of 2 minutes 20 s?, 2. Joseph jogs from one end A to the other end B of a straight, 300 m road in 2 minutes 30 seconds and then turns around, and jogs 100 m back to point C in another 1 minute. What are, Joseph’s average speeds and velocities in jogging (a) from A to, B and (b) from A to C?, 3. Abdul, while driving to school, computes the average speed for, his trip to be 20 km h–1. On his return trip along the same, route, there is less traf fic and the average speed is, 30 km h–1. What is the average speed for Abdul’s trip?, 4. A motorboat starting from rest on a lake accelerates in a straight, line at a constant rate of 3.0 m s–2 for 8.0 s. How far does the, boat travel during this time?, 5. A driver of a car travelling at 52 km h–1 applies the brakes and, accelerates uniformly in the opposite direction. The car stops, in 5 s. Another driver going at 3 km h–1 in another car applies, his brakes slowly and stops in 10 s. On the same graph paper,, plot the speed versus time graphs for the two cars. Which of, the two cars travelled farther after the brakes were applied?, 6. Fig 8.11 shows the distance-time graph of three objects A,B, and C. Study the graph and answer the following questions:, , Fig. 8.11, 112, , SCIENCE, , 2019-2020

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(a) Which of the three is travelling the fastest?, (b) Are all three ever at the same point on the road?, (c) How far has C travelled when B passes A?, (d) How far has B travelled by the time it passes C?, 7. A ball is gently dropped from a height of 20 m. If its velocity, increases uniformly at the rate of 10 m s-2, with what velocity, will it strike the ground? After what time will it strike the, ground?, 8. The speed-time graph for a car is shown is Fig. 8.12., , Fig. 8.12, , (a) Find how far does the car travel in the first 4 seconds., Shade the area on the graph that represents the distance, travelled by the car during the period., (b) Which part of the graph represents uniform motion of, the car?, 9. State which of the following situations are possible and give, an example for each of these:, (a) an object with a constant acceleration but with zero velocity, (b) an object moving with an acceleration but with uniform, speed., (c) an object moving in a certain direction with an, acceleration in the perpendicular direction., 10. An artificial satellite is moving in a circular orbit of radius, 42250 km. Calculate its speed if it takes 24 hours to revolve, around the earth., , 113, , MOTION, , 2019-2020

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Chapter 9, FORCE, , AND, , In the previous chapter, we described the, motion of an object along a straight line in, terms of its position, velocity and acceleration., We saw that such a motion can be uniform, or non-uniform. We have not yet discovered, what causes the motion. Why does the speed, of an object change with time? Do all motions, require a cause? If so, what is the nature of, this cause? In this chapter we shall make an, attempt to quench all such curiosities., For many centuries, the problem of, motion and its causes had puzzled scientists, and philosophers. A ball on the ground, when, given a small hit, does not move forever. Such, observations suggest that rest is the “natural, state” of an object. This remained the belief, until Galileo Galilei and Isaac Newton, developed an entirely different approach to, understand motion., , (a) The trolley moves along the, direction we push it., , LAWS, , OF, , MOTION, , In our everyday life we observe that some, effort is required to put a stationary object, into motion or to stop a moving object. We, ordinarily experience this as a muscular effort, and say that we must push or hit or pull on, an object to change its state of motion. The, concept of force is based on this push, hit or, pull. Let us now ponder about a ‘force’. What, is it? In fact, no one has seen, tasted or felt a, force. However, we always see or feel the effect, of a force. It can only be explained by, describing what happens when a force is, applied to an object. Pushing, hitting and, pulling of objects are all ways of bringing, objects in motion (Fig. 9.1). They move, because we make a force act on them., From your studies in earlier classes, you, are also familiar with the fact that a force can, be used to change the magnitude of velocity, of an object (that is, to make the object move, faster or slower) or to change its direction of, motion. We also know that a force can change, the shape and size of objects (Fig. 9.2)., , (b) The drawer is pulled., (a), , (b), (c) The hockey stick hits the ball forward, , Fig. 9.1: Pushing, pulling, or hitting objects change, their state of motion., , Fig. 9.2: (a) A spring expands on application of force;, (b) A spherical rubber ball becomes oblong, as we apply force on it.

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9.1 Balanced and Unbalanced, Forces, Fig. 9.3 shows a wooden block on a horizontal, table. Two strings X and Y are tied to the two, opposite faces of the block as shown. If we, apply a force by pulling the string X, the block, begins to move to the right. Similarly, if we, pull the string Y, the block moves to the left., But, if the block is pulled from both the sides, with equal forces, the block will not move., Such forces are called balanced forces and, do not change the state of rest or of motion of, an object. Now, let us consider a situation in, which two opposite forces of different, magnitudes pull the block. In this case, the, block would begin to move in the direction of, the greater force. Thus, the two forces are, not balanced and the unbalanced force acts, in the direction the block moves. This, suggests that an unbalanced force acting on, an object brings it in motion., , Fig. 9.3: Two forces acting on a wooden block, , What happens when some children try to, push a box on a rough floor? If they push the, , (a), , box with a small force, the box does not move, because of friction acting in a direction, opposite to the push [Fig. 9.4(a)]. This friction, force arises between two surfaces in contact;, in this case, between the bottom of the box, and floor’s rough surface. It balances the, pushing force and therefore the box does not, move. In Fig. 9.4(b), the children push the, box harder but the box still does not move., This is because the friction force still balances, the pushing force. If the children push the, box harder still, the pushing force becomes, bigger than the friction force [Fig. 9.4(c)]., There is an unbalanced force. So the box, starts moving., What happens when we ride a bicycle?, When we stop pedalling, the bicycle begins, to slow down. This is again because of the, friction forces acting opposite to the direction, of motion. In order to keep the bicycle moving,, we have to start pedalling again. It thus, appears that an object maintains its motion, under the continuous application of an, unbalanced force. However, it is quite, incorrect. An object moves with a uniform, velocity when the forces (pushing force and, frictional force) acting on the object are, balanced and there is no net external force, on it. If an unbalanced force is applied on, the object, there will be a change either in its, speed or in the direction of its motion. Thus,, to accelerate the motion of an object, an, unbalanced force is required. And the change, in its speed (or in the direction of motion), would continue as long as this unbalanced, force is applied. However, if this force is, , (b), , (c), , Fig. 9.4, FORCE, , AND, , LAWS, , OF, , MOTION, , 115

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removed completely, the object would, continue to move with the velocity it has, acquired till then., , 9.2 First Law of Motion, By observing the motion of objects on an, inclined plane Galileo deduced that objects, move with a constant speed when no force, acts on them. He observed that when a marble, rolls down an inclined plane, its velocity, increases [Fig. 9.5(a)]. In the next chapter,, you will learn that the marble falls under the, unbalanced force of gravity as it rolls down, and attains a definite velocity by the time it, reaches the bottom. Its velocity decreases, when it climbs up as shown in Fig. 9.5(b)., Fig. 9.5(c) shows a marble resting on an ideal, frictionless plane inclined on both sides., Galileo argued that when the marble is, released from left, it would roll down the slope, and go up on the opposite side to the same, height from which it was released. If the, inclinations of the planes on both sides are, equal then the marble will climb the same, distance that it covered while rolling down. If, the angle of inclination of the right-side plane, were gradually decreased, then the marble, would travel further distances till it reaches, the original height. If the right-side plane were, ultimately made horizontal (that is, the slope, is reduced to zero), the marble would continue, to travel forever trying to reach the same, height that it was released from. The, unbalanced forces on the marble in this case, are zero. It thus suggests that an unbalanced, (external) force is required to change the, motion of the marble but no net force is, needed to sustain the uniform motion of the, marble. In practical situations it is difficult, to achieve a zero unbalanced force. This is, because of the presence of the frictional force, acting opposite to the direction of motion., Thus, in practice the marble stops after, travelling some distance. The effect of the, frictional force may be minimised by using a, smooth marble and a smooth plane and, providing a lubricant on top of the planes., 116, , Fig. 9.5: (a) the downward motion; (b) the upward, motion of a marble on an inclined plane;, and (c) on a double inclined plane., , Newton further studied Galileo’s ideas on, force and motion and presented three, fundamental laws that govern the motion of, objects. These three laws are known as, Newton’s laws of motion. The first law of, motion is stated as:, An object remains in a state of rest or of, uniform motion in a straight line unless, compelled to change that state by an applied, force., In other words, all objects resist a change, in their state of motion. In a qualitative way,, the tendency of undisturbed objects to stay, at rest or to keep moving with the same, velocity is called inertia. This is why, the first, law of motion is also known as the law of, inertia., Certain experiences that we come across, while travelling in a motorcar can be, explained on the basis of the law of inertia., We tend to remain at rest with respect to the, seat until the driver applies a braking force, to stop the motorcar. With the application of, brakes, the car slows down but our body, tends to continue in the same state of motion, because of its inertia. A sudden application, of brakes may thus cause injury to us by, SCIENCE

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Galileo Galilei was born, on 15 February 1564 in, Pisa, Italy. Galileo, right, from his childhood, had, interest in mathematics, and natural philosophy., But, his, father, Vincenzo Galilei wanted, him to become a medical, doctor., Accordingly,, Galileo Galilei, Galileo enrolled himself, (1564 – 1642), for a medical degree at the, University of Pisa in 1581 which he never, completed because of his real interest in, mathematics. In 1586, he wrote his first, scientific book ‘The Little Balance [La, Balancitta]’, in which he described, Archimedes’ method of finding the relative, densities (or specific gravities) of substances, using a balance. In 1589, in his series of, essays – De Motu, he presented his theories, about falling objects using an inclined plane, to slow down the rate of descent., In 1592, he was appointed professor of, mathematics at the University of Padua in, the Republic of Venice. Here he continued his, observations on the theory of motion and, through his study of inclined planes and the, pendulum, formulated the correct law for, uniformly accelerated objects that the, distance the object moves is proportional to, the square of the time taken., Galileo was also a remarkable craftsman., He developed a series of telescopes whose, optical performance was much better than, that of other telescopes available during those, days. Around 1640, he designed the first, pendulum clock. In his book ‘Starry, Messenger’ on his astronomical discoveries,, Galileo claimed to have seen mountains on, the moon, the milky way made up of tiny, stars, and four small bodies orbiting Jupiter., In his books ‘Discourse on Floating Bodies’, and ‘Letters on the Sunspots’, he disclosed, his observations of sunspots., Using his own telescopes and through his, observations on Saturn and Venus, Galileo, argued that all the planets must orbit the Sun, and not the earth, contrary to what was, believed at that time., , FORCE, , AND, , LAWS, , OF, , MOTION, , impact or collision with the panels in front., Safety belts are worn to prevent such, accidents. Safety belts exert a force on our, body to make the forward motion slower. An, opposite experience is encountered when we, are standing in a bus and the bus begins to, move suddenly. Now we tend to fall, backwards. This is because the sudden start, of the bus brings motion to the bus as well, as to our feet in contact with the floor of the, bus. But the rest of our body opposes this, motion because of its inertia., When a motorcar makes a sharp turn at, a high speed, we tend to get thrown to one, side. This can again be explained on the basis, of the law of inertia. We tend to continue in, our straight-line motion. When an, unbalanced force is applied by the engine to, change the direction of motion of the, motorcar, we slip to one side of the seat due, to the inertia of our body., The fact that a body will remain at rest, unless acted upon by an unbalanced force, can be illustrated through the following, activities:, , Activity ______________ 9.1, •, •, , Make a pile of similar carom coins on, a table, as shown in Fig. 9.6., Attempt a sharp horizontal hit at the, bottom of the pile using another carom, coin or the striker. If the hit is strong, enough, the bottom coin moves out, quickly. Once the lowest coin is, removed, the inertia of the other coins, makes them ‘fall’ vertically on the table., , Fig. 9.6: Only the carom coin at the bottom of a, pile is removed when a fast moving carom, coin (or striker) hits it., 117

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Activity ______________ 9.2, •, , •, , •, , Set a five-rupee coin on a stiff card, covering an empty glass tumbler, standing on a table as shown in, Fig. 9.7., Give the card a sharp horizontal flick, with a finger. If we do it fast then the, card shoots away, allowing the coin to, fall vertically into the glass tumbler due, to its inertia., The inertia of the coin tries to maintain, its state of rest even when the card, flows off., , Fig. 9.7: When the card is flicked with the, finger the coin placed over it falls in the, tumbler., , Activity ______________ 9.3, •, •, •, , Place a water-filled tumbler on a tray., Hold the tray and turn around as fast, as you can., We observe that the water spills. Why?, , Observe that a groove is provided in a, saucer for placing the tea cup. It prevents, the cup from toppling over in case of sudden, jerks., , 9.3 Inertia and Mass, All the examples and activities given so far, illustrate that there is a resistance offered by, an object to change its state of motion. If it is, at rest it tends to remain at rest; if it is moving, it tends to keep moving. This property of an, object is called its inertia. Do all bodies have, the same inertia? We know that it is easier to, push an empty box than a box full of books., Similarly, if we kick a football it flies away., But if we kick a stone of the same size with, equal force, it hardly moves. We may, in fact,, get an injury in our foot while doing so!, Similarly, in activity 9.2, instead of a, 118, , five-rupees coin if we use a one-rupee coin, we, find that a lesser force is required to perform, the activity. A force that is just enough to, cause a small cart to pick up a large velocity, will produce a negligible change in the motion, of a train. This is because, in comparison to, the cart the train has a much lesser tendency, to change its state of motion. Accordingly, we, say that the train has more inertia than the, cart. Clearly, heavier or more massive objects, offer larger inertia. Quantitatively, the inertia, of an object is measured by its mass. We may, thus relate inertia and mass as follows:, Inertia is the natural tendency of an object to, resist a change in its state of motion or of, rest. The mass of an object is a measure of, its inertia., , Q, , uestions, 1. Which of the following has more, inertia: (a) a rubber ball and a, stone of the same size? (b) a, bicycle and a train? (c) a fiverupees coin and a one-rupee coin?, 2. In the following example, try to, identify the number of times the, velocity of the ball changes:, “A football player kicks a football, to another player of his team who, kicks the football towards the, goal. The goalkeeper of the, opposite team collects the football, and kicks it towards a player of, his own team”., Also identify the agent supplying, the force in each case., 3. Explain why some of the leaves, may get detached from a tree if, we vigorously shake its branch., 4. Why do you fall in the forward, direction when a moving bus, brakes to a stop and fall, backwards when it accelerates, from rest?, , 9.4 Second Law of Motion, The first law of motion indicates that when, an unbalanced external force acts on an, SCIENCE

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object, its velocity changes, that is, the object, gets an acceleration. We would now like to, study how the acceleration of an object, depends on the force applied to it and how, we measure a force. Let us recount some, observations from our everyday life. During, the game of table tennis if the ball hits a player, it does not hurt him. On the other hand, when, a fast moving cricket ball hits a spectator, it, may hurt him. A truck at rest does not require, any attention when parked along a roadside., But a moving truck, even at speeds as low as, 5 m s–1, may kill a person standing in its path., A small mass, such as a bullet may kill a, person when fired from a gun. These, observations suggest that the impact, produced by the objects depends on their, mass and velocity. Similarly, if an object is to, be accelerated, we know that a greater force, is required to give a greater velocity. In other, words, there appears to exist some quantity, of importance that combines the object’s, mass and its velocity. One such property, called momentum was introduced by Newton., The momentum, p of an object is defined as, the product of its mass, m and velocity, v., That is,, p = mv, (9.1), Momentum has both direction and, magnitude. Its direction is the same as that, of velocity, v. The SI unit of momentum is, kilogram-metre per second (kg m s-1). Since, the application of an unbalanced force brings, a change in the velocity of the object, it is, therefore clear that a force also produces a, change of momentum., Let us consider a situation in which a car, with a dead battery is to be pushed along a, straight road to give it a speed of 1 m s-1,, which is sufficient to start its engine. If one, or two persons give a sudden push, (unbalanced force) to it, it hardly starts. But, a continuous push over some time results in, a gradual acceleration of the car to this speed., It means that the change of momentum of, the car is not only determined by the, magnitude of the force but also by the time, during which the force is exerted. It may then, also be concluded that the force necessary to, FORCE, , AND, , LAWS, , OF, , MOTION, , change the momentum of an object depends, on the time rate at which the momentum is, changed., The second law of motion states that the, rate of change of momentum of an object is, proportional to the applied unbalanced force, in the direction of force., , 9.4.1 MATHEMATICAL, , FORMULATION OF, , SECOND LAW OF MOTION, , Suppose an object of mass, m is moving along, a straight line with an initial velocity, u. It is, uniformly accelerated to velocity, v in time, t, by the application of a constant force, F, throughout the time, t. The initial and final, momentum of the object will be, p1 = mu and, p2 = mv respectively., The change in momentum, , ∝ p2 – p1, ∝ mv – mu, ∝ m × (v – u)., , The rate of change of momentum ∝, , m × (v − u ), t, , Or, the applied force,, F∝, , m × (v − u ), t, , km × (v − u ), t, = kma, , F=, , (9.2), (9.3), , Here a [ = (v – u)/t ] is the acceleration,, which is the rate of change of velocity. The, quantity, k is a constant of proportionality., The SI units of mass and acceleration are kg, and m s-2 respectively. The unit of force is so, chosen that the value of the constant, k, becomes one. For this, one unit of force is, defined as the amount that produces an, acceleration of 1 m s-2 in an object of 1 kg, mass. That is,, 1 unit of force = k × (1 kg) × (1 m s-2)., Thus, the value of k becomes 1. From Eq. (9.3), (9.4), , F = ma, -2, , The unit of force is kg m s or newton,, which has the symbol N. The second law of, 119

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motion gives us a method to measure the, force acting on an object as a product of its, mass and acceleration., The second law of motion is often seen in, action in our everyday life. Have you noticed, that while catching a fast moving cricket ball,, a fielder in the ground gradually pulls his, hands backwards with the moving ball? In, doing so, the fielder increases the time during, which the high velocity of the moving ball, decreases to zero. Thus, the acceleration of, the ball is decreased and therefore the impact, of catching the fast moving ball (Fig. 9.8) is, also reduced. If the ball is stopped suddenly, then its high velocity decreases to zero in a, very short interval of time. Thus, the rate of, change of momentum of the ball will be large., Therefore, a large force would have to be, applied for holding the catch that may hurt, the palm of the fielder. In a high jump athletic, event, the athletes are made to fall either on, a cushioned bed or on a sand bed. This is to, increase the time of the athlete’s fall to stop, after making the jump. This decreases the, rate of change of momentum and hence the, force. Try to ponder how a karate player, breaks a slab of ice with a single blow., , The first law of motion can be, mathematically stated from the mathematical, expression for the second law of motion. Eq., (9.4) is, F = ma, , or, or, , F=, , m (v − u ), , (9.5), , t, , Ft = mv – mu, , That is, when F = 0, v = u for whatever time,, t is taken. This means that the object will, continue moving with uniform velocity, u, throughout the time, t. If u is zero then v will, also be zero. That is, the object will remain, at rest., Example 9.1 A constant force acts on an, object of mass 5 kg for a duration of, 2 s. It increases the object’s velocity, from 3 m s–1 to 7 m s -1. Find the, magnitude of the applied force. Now, if, the force was applied for a duration of, 5 s, what would be the final velocity of, the object?, Solution:, We have been given that u = 3 m s–1, and v = 7 m s-1, t = 2 s and m = 5 kg., From Eq. (9.5) we have,, F =, , m (v − u ), t, , Substitution of values in this relation, gives, F = 5 kg (7 m s-1 – 3 m s-1)/2 s = 10 N., Now, if this force is applied for a, duration of 5 s (t = 5 s), then the final, velocity can be calculated by rewriting, Eq. (9.5) as, v=u+, , Fig. 9.8: A fielder pulls his hands gradually with the, moving ball while holding a catch., 120, , Ft, m, , On substituting the values of u, F, m, and t, we get the final velocity,, v = 13 m s-1., , SCIENCE

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Example 9.2 Which would require a, greater force –– accelerating a 2 kg mass, at 5 m s–2 or a 4 kg mass at 2 m s-2 ?, Solution:, From Eq. (9.4), we have F = ma., Here we have m1 = 2 kg; a1 = 5 m s-2, and m2 = 4 kg; a2 = 2 m s-2., Thus, F1 = m1a1 = 2 kg × 5 m s-2 = 10 N;, and F2 = m2a2 = 4 kg × 2 m s-2 = 8 N., ⇒ F1 > F2., Thus, accelerating a 2 kg mass at, 5 m s-2 would require a greater force., Example 9.3 A motorcar is moving with a, velocity of 108 km/h and it takes 4 s to, stop after the brakes are applied., Calculate the force exerted by the, brakes on the motorcar if its mass along, with the passengers is 1000 kg., , Solution:, From Eq. (9.4) we have m1 = F/a1; and, m2 = F/a2. Here, a1 = 10 m s-2;, a2 = 20 m s-2 and F = 5 N., Thus, m1 = 5 N/10 m s-2 = 0.50 kg; and, m2 = 5 N/20 m s-2 = 0.25 kg., If the two masses were tied together,, the total mass, m would be, m = 0.50 kg + 0.25 kg = 0.75 kg., The acceleration, a produced in the, combined mass by the 5 N force would, be, a = F/m = 5 N/0.75 kg = 6.67 m s-2., Example 9.5 The velocity-time graph of a, ball of mass 20 g moving along a, straight line on a long table is given in, Fig. 9.9., , Solution:, The initial velocity of the motorcar, u = 108 km/h, = 108 × 1000 m/(60 × 60 s), = 30 m s-1, and the final velocity of the motorcar, v = 0 m s-1., The total mass of the motorcar along, with its passengers = 1000 kg and the, time taken to stop the motorcar, t = 4 s., From Eq. (9.5) we have the magnitude, of the force (F) applied by the brakes as, m(v – u)/t., On substituting the values, we get, F = 1000 kg × (0 – 30) m s-1/4 s, = – 7500 kg m s-2 or – 7500 N., The negative sign tells us that the force, exerted by the brakes is opposite to the, direction of motion of the motorcar., Example 9.4 A force of 5 N gives a mass, m1, an acceleration of 10 m s–2 and a, mass m2, an acceleration of 20 m s-2., What acceleration would it give if both, the masses were tied together?, FORCE, , AND, , LAWS, , OF, , MOTION, , Fig. 9.9, , How much force does the table exert, on the ball to bring it to rest?, Solution:, The initial velocity of the ball is 20 cm s-1., Due to the frictional force exerted by the, table, the velocity of the ball decreases, down to zero in 10 s. Thus, u = 20 cm s–1;, v = 0 cm s-1 and t = 10 s. Since the, velocity-time graph is a straight line, it is, clear that the ball moves with a constant, acceleration. The acceleration a is, v −u, t, = (0 cm s-1 – 20 cm s-1)/10 s, = –2 cm s-2 = –0.02 m s-2., , a =, , 121

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The force exerted on the ball F is,, F = ma = (20/1000) kg × (– 0.02 m s-2), = – 0.0004 N., The negative sign implies that the, frictional force exerted by the table is, opposite to the direction of motion of, the ball., , 9.5 Third Law of Motion, The first two laws of motion tell us how an, applied force changes the motion and provide, us with a method of determining the force., The third law of motion states that when one, object exerts a force on another object, the, second object instantaneously exerts a force, back on the first. These two forces are always, equal in magnitude but opposite in direction., These forces act on different objects and never, on the same object. In the game of football, sometimes we, while looking at the football, and trying to kick it with a greater force,, collide with a player of the opposite team., Both feel hurt because each applies a force, to the other. In other words, there is a pair of, forces and not just one force. The two, opposing forces are also known as action and, reaction forces., Let us consider two spring balances, connected together as shown in Fig. 9.10. The, fixed end of balance B is attached with a rigid, support, like a wall. When a force is applied, through the free end of spring balance A, it is, observed that both the spring balances show, the same readings on their scales. It means, that the force exerted by spring balance A on, balance B is equal but opposite in direction, to the force exerted by the balance B on, balance A. Any of these two forces can be called, as action and the other as reaction. This gives, us an alternative statement of the third law of, motion i.e., to every action there is an equal, and opposite reaction. However, it must be, remembered that the action and reaction, always act on two different objects,, simultaneously., , 122, , Fig. 9.10: Action and reaction forces are equal and, opposite., , Suppose you are standing at rest and, intend to start walking on a road. You must, accelerate, and this requires a force in, accordance with the second law of motion., Which is this force? Is it the muscular effort, you exert on the road? Is it in the direction, we intend to move? No, you push the road, below backwards. The road exerts an equal, and opposite force on your feet to make you, move forward., It is important to note that even though, the action and reaction forces are always, equal in magnitude, these forces may not, produce accelerations of equal magnitudes., This is because each force acts on a different, object that may have a different mass., When a gun is fired, it exerts a forward, force on the bullet. The bullet exerts an equal, and opposite force on the gun. This results in, the recoil of the gun (Fig. 9.11). Since the gun, has a much greater mass than the bullet, the, acceleration of the gun is much less than the, acceleration of the bullet. The third law of, motion can also be illustrated when a sailor, jumps out of a rowing boat. As the sailor, jumps forward, the force on the boat moves it, backwards (Fig. 9.12)., , Fig. 9.11: A forward force on the bullet and recoil of, the gun., , SCIENCE

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The cart shown in this activity can be, constructed by using a 12 mm or 18 mm thick, plywood board of about 50 cm × 100 cm with, two pairs of hard ball-bearing wheels (skate, wheels are good to use). Skateboards are not, as effective because it is difficult to maintain, straight-line motion., , 9.6 Conservation of Momentum, Fig. 9.12: As the sailor jumps in forward direction,, the boat moves backwards., , Activity ______________ 9.4, •, •, , •, , •, , Request two children to stand on two, separate carts as shown in Fig. 9.13., Give them a bag full of sand or some, other heavy object. Ask them to play a, game of catch with the bag., Does each of them experience an, instantaneous force as a result of, throwing the sand bag?, You can paint a white line on, cartwheels to observe the motion of the, two carts when the children throw the, bag towards each other., , Suppose two objects (two balls A and B, say), of masses mA and mB are travelling in the same, direction along a straight line at different, velocities uA and uB, respectively [Fig. 9.14(a)]., And there are no other external unbalanced, forces acting on them. Let uA > uB and the, two balls collide with each other as shown in, Fig. 9.14(b). During collision which lasts for, a time t, the ball A exerts a force FAB on ball B, and the ball B exerts a force FBA on ball A., Suppose vA and vB are the velocities of the, two balls A and B after the collision,, respectively [Fig. 9.14(c)]., , Fig. 9.14: Conservation of momentum in collision of, two balls., , From Eq. (9.1), the momenta (plural of, momentum) of ball A before and after the, collision are mAuA and mAvA, respectively. The, rate of change of its momentum (or FAB), during the collision will be m A, Fig. 9.13, , Now, place two children on one cart and, one on another cart. The second law of motion, can be seen, as this arrangement would show, different accelerations for the same force., FORCE, , AND, , LAWS, , OF, , MOTION, , (v A − u A ), t, , ., , Similarly, the rate of change of momentum of, ball B (= FBA) during the collision will be, mB, , (v B − u B ), t, , ., , According to the third law of motion,, the force FAB exerted by ball A on ball B, 123

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and the force FBA exerted by the ball B on ball, A must be equal and opposite to each other., Therefore,, FAB = – FBA, (9.6), mA, , or, , (v A − u A ), t, , = – mB, , (v B − u B ), ., t, , Activity ______________ 9.6, •, , •, , This gives,, mAuA + mBuB = mAvA + mBvB, , (9.7), , Since (mAuA + mBuB) is the total momentum, of the two balls A and B before the collision, and (mAvA + mBvB) is their total momentum, after the collision, from Eq. (9.7) we observe, that the total momentum of the two balls, remains unchanged or conserved provided no, other external force acts., As a result of this ideal collision, experiment, we say that the sum of momenta, of the two objects before collision is equal to, the sum of momenta after the collision, provided there is no external unbalanced, force acting on them. This is known as the, law of conservation of momentum. This, statement can alternatively be given as the, total momentum of the two objects is, unchanged or conserved by the collision., , •, •, , Take a test tube of good quality glass, material and put a small amount of, water in it. Place a stop cork at the, mouth of it., Now suspend the test tube horizontally, by two strings or wires as shown in, Fig. 9.16., Heat the test tube with a burner until, water vaporises and the cork blows out., Observe that the test tube recoils in, the direction opposite to the direction, of the cork., , Activity ______________ 9.5, •, , •, , •, , •, , •, , Take a big rubber balloon and inflate, it fully. Tie its neck using a thread. Also, using adhesive tape, fix a straw on the, surface of this balloon., Pass a thread through the straw and, hold one end of the thread in your hand, or fix it on the wall., Ask your friend to hold the other end, of the thread or fix it on a wall at some, distance. This arrangement is shown, in Fig. 9.15., Now remove the thread tied on the neck, of balloon. Let the air escape from the, mouth of the balloon., Observe the direction in which the, straw moves., , Fig. 9.15, 124, , Fig. 9.16, , •, , Also, observe the difference in the, velocity the cork appears to have and, that of the recoiling test tube., , Example 9.6 A bullet of mass 20 g is, horizontally fired with a velocity, 150 m s-1 from a pistol of mass 2 kg., What is the recoil velocity of the pistol?, Solution:, We have the mass of bullet,, m1 = 20 g (= 0.02 kg) and the mass of, the pistol, m2 = 2 kg; initial velocities of, the bullet (u 1) and pistol (u 2) = 0,, respectively. The final velocity of the, bullet, v1 = + 150 m s-1. The direction, of bullet is taken from left to right, (positive, by convention, Fig. 9.17). Let, v be the recoil velocity of the pistol., SCIENCE

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Total momenta of the pistol and bullet, before the fire, when the gun is at rest, = (2 + 0.02) kg × 0 m s–1, = 0 kg m s–1, Total momenta of the pistol and bullet, after it is fired, = 0.02 kg × (+ 150 m s–1), + 2 kg × v m s–1, = (3 + 2v) kg m s–1, According to the law of conservation of, momentum, Total momenta after the fire = Total, momenta before the fire, 3 + 2v = 0, ⇒ v = − 1.5 m s–1., Negative sign indicates that the, direction in which the pistol would, recoil is opposite to that of bullet, that, is, right to left., , Example 9.7 A girl of mass 40 kg jumps, with a horizontal velocity of 5 m s-1 onto, a stationary cart with frictionless, wheels. The mass of the cart is 3 kg., What is her velocity as the cart starts, moving? Assume that there is no, external unbalanced force working in, the horizontal direction., Solution:, Let v be the velocity of the girl on the, cart as the cart starts moving., The total momenta of the girl and cart, before the interaction, =, =, , 40 kg × 5 m s–1 + 3 kg × 0 m s–1, 200 kg m s–1., , Total momenta after the interaction, =, =, , (40 + 3) kg × v m s–1, 43 v kg m s–1., , According to the law of conservation of, momentum, the total momentum is, conserved during the interaction., That is,, 43 v = 200, ⇒ v = 200/43 = + 4.65 m s–1., , Fig. 9.17: Recoil of a pistol, , The girl on cart would move with a, velocity of 4.65 m s–1 in the direction in, which the girl jumped (Fig. 9.18)., , (a), , (b), Fig. 9.18: The girl jumps onto the cart., , FORCE, , AND, , LAWS, , OF, , MOTION, , 125

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If v is the velocity of the two entangled, players after the collision, the total, momentum then, = (m1 + m2) × v, = (60 + 55) kg × v m s–1, = 115 × v kg m s–1., Equating the momenta of the system, before and after collision, in accordance, with the law of conservation of, momentum, we get, v = – 30/115, = – 0.26 m s–1., Thus, the two entangled players would, move with velocity 0.26 m s–1 from right, to left, that is, in the direction the, second player was moving before, the collision., , Example 9.8 Two hockey players of, opposite teams, while trying to hit a, hockey ball on the ground collide and, immediately become entangled. One, has a mass of 60 kg and was moving, with a velocity 5.0 m s–1 while the other, has a mass of 55 kg and was moving, faster with a velocity 6.0 m s–1 towards, the first player. In which direction and, with what velocity will they move after, they become entangled? Assume that, the frictional force acting between the, feet of the two players and ground is, negligible., Solution:, , Fig. 9.19: A collision of two hockey players: (a) before collision and (b) after collision., , Let the first player be moving from left, to right. By convention left to right is, taken as the positive direction and thus, right to left is the negative direction (Fig., 9.19). If symbols m and u represent the, mass and initial velocity of the two, players, respectively. Subscripts 1 and, 2 in these physical quantities refer to, the two hockey players. Thus,, m1 = 60 kg; u1 = + 5 m s-1; and, m2 = 55 kg; u2 = – 6 m s-1., The total momentum of the two players, before the collision, = 60 kg × (+ 5 m s-1) +, 55 kg × (– 6 m s-1), = – 30 kg m s-1, 126, , Q, , uestions, 1. If action is always equal to the, reaction, explain how a horse can, pull a cart., 2. Explain, why is it difficult for a, fireman to hold a hose, which, ejects large amounts of water at, a high velocity., 3. From a rifle of mass 4 kg, a bullet, of mass 50 g is fired with an, initial velocity of 35 m s –1 ., Calculate the initial recoil velocity, of the rifle., SCIENCE

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4. Two objects of masses 100 g and, 200 g are moving along the same, line and direction with velocities, of 2 m s–1 and 1 m s–1, respectively., , They collide and after the, collision, the first object moves at, a velocity of 1.67 m s–1. Determine, the velocity of the second object., , CONSERVATION LAWS, All conservation laws such as conservation of momentum, energy, angular momentum,, charge etc. are considered to be fundamental laws in physics. These are based on, observations and experiments. It is important to remember that a conservation law cannot, be proved. It can be verified, or disproved, by experiments. An experiment whose result is, in conformity with the law verifies or substantiates the law; it does not prove the law. On, the other hand, a single experiment whose result goes against the law is enough to disprove, it., The law of conservation of momentum has been deduced from large number of, observations and experiments. This law was formulated nearly three centuries ago. It is, interesting to note that not a single situation has been realised so far, which contradicts, this law. Several experiences of every-day life can be explained on the basis of the law of, conservation of momentum., , What, you have, learnt, •, , •, •, •, •, , •, , •, , •, •, FORCE, , AND, , LAWS, , OF, , MOTION, , First law of motion: An object continues to be in a state of rest, or of uniform motion along a straight line unless acted upon, by an unbalanced force., The natural tendency of objects to resist a change in their state, of rest or of uniform motion is called inertia., The mass of an object is a measure of its inertia. Its SI unit is, kilogram (kg)., Force of friction always opposes motion of objects., Second law of motion: The rate of change of momentum of an, object is proportional to the applied unbalanced force in the, direction of the force., The SI unit of force is kg m s–2. This is also known as newton, and represented by the symbol N. A force of one newton, produces an acceleration of 1 m s–2 on an object of mass 1 kg., The momentum of an object is the product of its mass and, velocity and has the same direction as that of the velocity. Its, SI unit is kg m s–1., Third law of motion: To every action, there is an equal and, opposite reaction and they act on two different bodies., In an isolated system (where there is no external force), the, total momentum remains conserved., 127

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Exercises, 1. An object experiences a net zero external unbalanced force. Is, it possible for the object to be travelling with a non-zero velocity?, If yes, state the conditions that must be placed on the, magnitude and direction of the velocity. If no, provide a reason., 2. When a carpet is beaten with a stick, dust comes out of it., Explain., 3. Why is it advised to tie any luggage kept on the roof of a bus, with a rope?, 4. A batsman hits a cricket ball which then rolls on a level ground., After covering a short distance, the ball comes to rest. The ball, slows to a stop because, (a) the batsman did not hit the ball hard enough., (b) velocity is proportional to the force exerted on the ball., (c) there is a force on the ball opposing the motion., (d) there is no unbalanced force on the ball, so the ball would, want to come to rest., 5. A truck starts from rest and rolls down a hill with a constant, acceleration. It travels a distance of 400 m in 20 s. Find its, acceleration. Find the force acting on it if its mass is, 7 tonnes (Hint: 1 tonne = 1000 kg.), 6. A stone of 1 kg is thrown with a velocity of 20 m s–1 across, the frozen surface of a lake and comes to rest after travelling, a distance of 50 m. What is the force of friction between the, stone and the ice?, 7. A 8000 kg engine pulls a train of 5 wagons, each of 2000 kg,, along a horizontal track. If the engine exerts a force of 40000 N, and the track offers a friction force of 5000 N, then calculate:, (a) the net accelerating force and, (b) the acceleration of the train., 8. An automobile vehicle has a mass of 1500 kg. What must, be the force between the vehicle and road if the vehicle is to, be stopped with a negative acceleration of 1.7 m s–2?, 9. What is the momentum of an object of mass m, moving with a, velocity v?, (a) (mv)2, (b) mv2, (c) ½ mv2, (d) mv, 10. Using a horizontal force of 200 N, we intend to move a wooden, cabinet across a floor at a constant velocity. What is the friction, force that will be exerted on the cabinet?, 11. Two objects, each of mass 1.5 kg, are moving in the same, straight line but in opposite directions. The velocity of each, object is 2.5 m s-1 before the collision during which they, 128, , SCIENCE

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12., , 13., , 14., , 15., , 16., , 17., , 18., , FORCE, , AND, , LAWS, , OF, , MOTION, , stick together. What will be the velocity of the combined, object after collision?, According to the third law of motion when we push on an object,, the object pushes back on us with an equal and opposite force., If the object is a massive truck parked along the roadside, it, will probably not move. A student justifies this by answering, that the two opposite and equal forces cancel each other., Comment on this logic and explain why the truck does not, move., A hockey ball of mass 200 g travelling at 10 m s–1 is struck by, a hockey stick so as to return it along its original path with a, velocity at 5 m s–1. Calculate the magnitude of change of, momentum occurred in the motion of the hockey ball by the, force applied by the hockey stick., A bullet of mass 10 g travelling horizontally with a velocity of, 150 m s–1 strikes a stationary wooden block and comes to rest, in 0.03 s. Calculate the distance of penetration of the bullet, into the block. Also calculate the magnitude of the force exerted, by the wooden block on the bullet., An object of mass 1 kg travelling in a straight line with a velocity, of 10 m s–1 collides with, and sticks to, a stationary wooden, block of mass 5 kg. Then they both move off together in the, same straight line. Calculate the total momentum just before, the impact and just after the impact. Also, calculate the velocity, of the combined object., An object of mass 100 kg is accelerated uniformly from a velocity, of 5 m s–1 to 8 m s–1 in 6 s. Calculate the initial and final, momentum of the object. Also, find the magnitude of the force, exerted on the object., Akhtar, Kiran and Rahul were riding in a motorcar that was, moving with a high velocity on an expressway when an insect, hit the windshield and got stuck on the windscreen. Akhtar, and Kiran started pondering over the situation. Kiran suggested, that the insect suffered a greater change in momentum as, compared to the change in momentum of the motorcar (because, the change in the velocity of the insect was much more than, that of the motorcar). Akhtar said that since the motorcar was, moving with a larger velocity, it exerted a larger force on the, insect. And as a result the insect died. Rahul while putting an, entirely new explanation said that both the motorcar and the, insect experienced the same force and a change in their, momentum. Comment on these suggestions., How much momentum will a dumb-bell of mass 10 kg transfer, to the floor if it falls from a height of 80 cm? Take its downward, acceleration to be 10 m s–2., , 129

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Additional, Exercises, A1. The following is the distance-time table of an object in motion:, Time in seconds, Distance in metres, 0, 0, 1, 1, 2, 8, 3, 27, 4, 64, 5, 125, 6, 216, 7, 343, (a) What conclusion can you draw about the acceleration?, Is it constant, increasing, decreasing, or zero?, (b) What do you infer about the forces acting on the object?, A2. Two persons manage to push a motorcar of mass 1200 kg at a, uniform velocity along a level road. The same motorcar can be, pushed by three persons to produce an acceleration of, 0.2 m s-2. With what force does each person push the motorcar?, (Assume that all persons push the motorcar with the same, muscular effort.), A3. A hammer of mass 500 g, moving at 50 m s-1, strikes a nail., The nail stops the hammer in a very short time of 0.01 s. What, is the force of the nail on the hammer?, A4. A motorcar of mass 1200 kg is moving along a straight line, with a uniform velocity of 90 km/h. Its velocity is slowed down, to 18 km/h in 4 s by an unbalanced external force. Calculate, the acceleration and change in momentum. Also calculate the, magnitude of the force required., , 130, , SCIENCE

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Chapter 10, GRAVITATION, In Chapters 8 and 9, we have learnt about, the motion of objects and force as the cause, of motion. We have learnt that a force is, needed to change the speed or the direction, of motion of an object. We always observe that, an object dropped from a height falls towards, the earth. We know that all the planets go, around the Sun. The moon goes around the, earth. In all these cases, there must be some, force acting on the objects, the planets and, on the moon. Isaac Newton could grasp that, the same force is responsible for all these., This force is called the gravitational force., In this chapter we shall learn about, gravitation and the universal law of, gravitation. We shall discuss the motion of, objects under the influence of gravitational, force on the earth. We shall study how the, weight of a body varies from place to place., We shall also discuss the conditions for, objects to float in liquids., , Let us try to understand the motion of, the moon by recalling activity 8.11., , Activity _____________ 10.1, •, •, , •, •, •, , Take a piece of thread., Tie a small stone at one end. Hold the, other end of the thread and whirl it, round, as shown in Fig. 10.1., Note the motion of the stone., Release the thread., Again, note the direction of motion of, the stone., , 10.1 Gravitation, We know that the moon goes around the, earth. An object when thrown upwards,, reaches a certain height and then falls, downwards. It is said that when Newton was, sitting under a tree, an apple fell on him. The, fall of the apple made Newton start thinking., He thought that: if the earth can attract an, apple, can it not attract the moon? Is the force, the same in both cases? He conjectured that, the same type of force is responsible in both, the cases. He argued that at each point of its, orbit, the moon falls towards the earth,, instead of going off in a straight line. So, it, must be attracted by the earth. But we do, not really see the moon falling towards the, earth., , Fig. 10.1: A stone describing a circular path with a, velocity of constant magnitude., , Before the thread is released, the stone, moves in a circular path with a certain speed, and changes direction at every point. The, change in direction involves change in velocity, or acceleration. The force that causes this, acceleration and keeps the body moving along, the circular path is acting towards the centre., This force is called the centripetal (meaning, ‘centre-seeking’) force. In the absence of this, , 2019-2020

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force, the stone flies off along a straight line., This straight line will be a tangent to the, circular path., , More to know, , Tangent to a circle, , A straight line that meets the circle, at one and only one point is called a, tangent to the circle. Straight line, ABC is a tangent to the circle at, point B., , 10.1.1 UNIVERSAL LAW OF GRAVITATION, Every object in the universe attracts every, other object with a force which is proportional, to the product of their masses and inversely, proportional to the square of the distance, between them. The force is along the line, joining the centres of two objects., , F = G, , Mm, d, , The motion of the moon around the earth, is due to the centripetal force. The centripetal, force is provided by the force of attraction of, the earth. If there were no such force, the, moon would pursue a uniform straight line, motion., It is seen that a falling apple is attracted, towards the earth. Does the apple attract the, earth? If so, we do not see the earth moving, towards an apple. Why?, According to the third law of motion, the, apple does attract the earth. But according, to the second law of motion, for a given force,, acceleration is inversely proportional to the, mass of an object [Eq. (9.4)]. The mass of an, apple is negligibly small compared to that of, the earth. So, we do not see the earth moving, towards the apple. Extend the same argument, for why the earth does not move towards the, moon., In our solar system, all the planets go, around the Sun. By arguing the same way,, we can say that there exists a force between, the Sun and the planets. From the above facts, Newton concluded that not only does the, earth attract an apple and the moon, but all, objects in the universe attract each other. This, force of attraction between objects is called, the gravitational force., , 2, , Fig. 10.2: The gravitational force between two, uniform objects is directed along the line, joining their centres., , Let two objects A and B of masses M and, m lie at a distance d from each other as shown, in Fig. 10.2. Let the force of attraction between, two objects be F. According to the universal, law of gravitation, the force between two, objects is directly proportional to the product, of their masses. That is,, (10.1), F∝M × m, And the force between two objects is inversely, proportional to the square of the distance, between them, that is,, F∝, , 1, , (10.2), , d2, , Combining Eqs. (10.1) and (10.2), we get, F, , ∝, , M ×m, , or, F = G, , (10.3), , d2, , M×m, d, , 2, , (10.4), , where G is the constant of proportionality and, is called the universal gravitation constant., By multiplying crosswise, Eq. (10.4) gives, F×d2=GM×m, , 132, , SCIENCE, , 2019-2020

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Isaac Newton was born, in Woolsthorpe near, Grantham, England., He, is generally, regarded as the most, original, and, influential theorist in, the history of science., He was born in a poor, farming family. But he, was not good at, farming. He was sent, to study at Cambridge, Isaac Newton, University in 1661. In, (1642 – 1727), 1665 a plague broke, out in Cambridge and so Newton took a year, off. It was during this year that the incident of, the apple falling on him is said to have, occurred. This incident prompted Newton to, explore the possibility of connecting gravity, with the force that kept the moon in its orbit., This led him to the universal law of, gravitation. It is remarkable that many great, scientists before him knew of gravity but failed, to realise it., Newton formulated the well-known laws of, motion. He worked on theories of light and, colour. He designed an astronomical telescope, to carry out astronomical observations., Newton was also a great mathematician. He, invented a new branch of mathematics, called, calculus. He used it to prove that for objects, outside a sphere of uniform density, the sphere, behaves as if the whole of its mass is, concentrated at its centre. Newton, transfor med the structure of physical, science with his three laws of motion and the, universal law of gravitation. As the keystone, of the scientific revolution of the seventeenth, century, Newton’s work combined the, contributions of Copernicus, Kepler, Galileo,, and others into a new powerful synthesis., It is remarkable that though the, gravitational theory could not be verified at, that time, there was hardly any doubt about, its correctness. This is because Newton based, his theory on sound scientific reasoning and, backed it with mathematics. This made the, theory simple and elegant. These qualities are, now recognised as essential requirements of a, good scientific theory., , How did Newton guess the, inverse-square rule?, There has always been a great interest, in the motion of planets. By the 16th, century, a lot of data on the motion of, planets had been collected by many, astronomers. Based on these data, Johannes Kepler derived three laws,, which govern the motion of planets., These are called Kepler’s laws. These are:, 1. The orbit of a planet is an ellipse with, the Sun at one of the foci, as shown in, the figure given below. In this figure O, is the position of the Sun., 2. The line joining the planet and the Sun, sweep equal areas in equal intervals, of time. Thus, if the time of travel from, A to B is the same as that from C to D,, then the areas OAB and OCD are, equal., 3. The cube of the mean distance of a, planet from the Sun is proportional to, the square of its orbital period T. Or,, r3/T2 = constant., It is important to note that Kepler, could not give a theory to explain, the motion of planets. It was Newton, who showed that the cause of the, planetary motion is the gravitational, force that the Sun exerts on them. Newton, used the third law, C, D, of Kepler to, calculate, the, gravitational force, B, O, of attraction. The, A, gravitational force, of the earth is, weakened by distance. A simple argument, goes like this. We can assume that the, planetary orbits are circular. Suppose the, orbital velocity is v and the radius of the, orbit is r. Then the force acting on an, orbiting planet is given by F ∝ v2/r., If T denotes the period, then v = 2πr/T,, so that v2 ∝ r2/T2., We can rewrite this as v2 ∝ (1/r) ×, 3, ( r /T2). Since r3/T2 is constant by Kepler’s, third law, we have v2 ∝ 1/r. Combining, this with F ∝ v2/ r, we get, F ∝ 1/ r2., , 133, , GRAVITATION, , 2019-2020

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or, , G=, , Fd, , (10.5), , M×m, , The SI unit of G can be obtained by, substituting the units of force, distance and, mass in Eq. (10.5) as N m2 kg–2., The value of G was found out by, Henry Cavendish (1731 – 1810) by using a, sensitive balance. The accepted value of G is, 6.673 × 10–11 N m2 kg–2., We know that there exists a force of, attraction between any two objects. Compute, the value of this force between you and your, friend sitting closeby. Conclude how you do, not experience this force!, , More to know, , From Eq. (10.4), the force exerted by the, earth on the moon is, , 2, , The law is universal in the sense that, it is applicable to all bodies, whether, the bodies are big or small, whether, they are celestial or terrestrial., Inverse-square, Saying that F is inversely, proportional to the square of d, means, for example, that if d gets, bigger by a factor of 6, F becomes, 1, times smaller., 36, , Example 10.1 The mass of the earth is, 6 × 1024 kg and that of the moon is, 7.4 × 1022 kg. If the distance between the, earth and the moon is 3.84×105 km,, calculate the force exerted by the earth on, the moon. (Take G = 6.7 × 10–11 N m2 kg-2), Solution:, The mass of the earth, M = 6 × 1024 kg, The mass of the moon,, m = 7.4 × 1022 kg, The distance between the earth and the, moon,, d = 3.84 × 105 km, = 3.84 × 105 × 1000 m, = 3.84 × 108 m, G = 6.7 × 10–11 N m2 kg–2, , F =G, , =, , M ×m, d2, , 6.7 × 10 −11 N m 2 kg -2 × 6 × 1024 kg × 7.4 × 1022 kg, (3.84 × 108 m)2, , = 2.02 × 1020 N., , Thus, the force exerted by the earth on, the moon is 2.02 × 1020 N., , Q, , uestions, 1. State the universal law of, gravitation., 2. Write the formula to find the, magnitude of the gravitational, force between the earth and an, object on the surface of the earth., , 10.1.2 IMPORTANCE, , OF THE UNIVERSAL, , LAW OF GRAVITATION, , The universal law of gravitation successfully, explained several phenomena which were, believed to be unconnected:, (i) the force that binds us to the earth;, (ii) the motion of the moon around the, earth;, (iii) the motion of planets around the Sun;, and, (iv) the tides due to the moon and the Sun., , 10.2 Free Fall, Let us try to understand the meaning of free, fall by performing this activity., , Activity _____________ 10.2, •, •, •, , Take a stone., Throw it upwards., It reaches a certain height and then it, starts falling down., , We have learnt that the earth attracts, objects towards it. This is due to the, gravitational force. Whenever objects fall, towards the earth under this force alone, we, say that the objects are in free fall. Is there any, , 134, , SCIENCE, , 2019-2020

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change in the velocity of falling objects? While, falling, there is no change in the direction of, motion of the objects. But due to the earth’s, attraction, there will be a change in the, magnitude of the velocity. Any change in, velocity involves acceleration. Whenever an, object falls towards the earth, an acceleration, is involved. This acceleration is due to the, earth’s gravitational force. Therefore, this, acceleration is called the acceleration due to, the gravitational force of the earth (or, acceleration due to gravity). It is denoted by, g. The unit of g is the same as that of, acceleration, that is, m s–2., We know from the second law of motion, that force is the product of mass and, acceleration. Let the mass of the stone in, activity 10.2 be m. We already know that there, is acceleration involved in falling objects due, to the gravitational force and is denoted by g., Therefore the magnitude of the gravitational, force F will be equal to the product of mass, and acceleration due to the gravitational, force, that is,, F=mg, (10.6), From Eqs. (10.4) and (10.6) we have, mg =G, , g= G, , g= G, , 2, , =, , 2, , 2, , 6.7 × 10-11 N m 2 kg -2 × 6 × 1024 kg, (6.4 × 106 m)2, , = 9.8 m s–2., , Thus, the value of acceleration due to gravity, of the earth, g = 9.8 m s–2., , 10.2.2 MOTION, , OF OBJECTS UNDER THE, , FORCE OF THE EARTH, , 2, , M, , Let us do an activity to understand whether, all objects hollow or solid, big or small, will, fall from a height at the same rate., , Activity _____________ 10.3, •, , •, , (10.8), , M, R, , M, R, , M ×m, R, , g=G, , To calculate the value of g, we should put, the values of G, M and R in Eq. (10.9),, namely, universal gravitational constant,, G = 6.7 × 10–11 N m2 kg-2, mass of the earth,, M = 6 × 1024 kg, and radius of the earth,, R = 6.4 × 106 m., , INFLUENCE OF GRAVITATIONAL, , (10.7), 2, d, where M is the mass of the earth, and d is, the distance between the object and the earth., Let an object be on or near the surface of, the earth. The distance d in Eq. (10.7) will be, equal to R, the radius of the earth. Thus, for, objects on or near the surface of the earth,, mg = G, , 10.2.1 TO CALCULATE THE VALUE OF g, , M ×m, d, , or, , calculations, we can take g to be more or less, constant on or near the earth. But for objects, far from the earth, the acceleration due to, gravitational force of earth is given by, Eq. (10.7)., , (10.9), , The earth is not a perfect sphere. As the, radius of the earth increases from the poles, to the equator, the value of g becomes greater, at the poles than at the equator. For most, , Take a sheet of paper and a stone. Drop, them simultaneously from the first floor, of a building. Observe whether both of, them reach the ground simultaneously., We see that paper reaches the ground, little later than the stone. This happens, because of air resistance. The air offers, resistance due to friction to the motion, of the falling objects. The resistance, offered by air to the paper is more than, the resistance offered to the stone. If, we do the experiment in a glass jar from, which air has been sucked out, the, paper and the stone would fall at the, same rate., 135, , GRAVITATION, , 2019-2020

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u +v, 2, = (0 m s–1+ 5 m s–1)/2, = 2.5 m s–1, (iii) distance travelled, s = ½ a t2, = ½ × 10 m s–2 × (0.5 s)2, = ½ × 10 m s–2 × 0.25 s2, = 1.25 m, , We know that an object experiences, acceleration during free fall. From Eq. (10.9),, this acceleration experienced by an object is, independent of its mass. This means that all, objects hollow or solid, big or small, should, fall at the same rate. According to a story,, Galileo dropped different objects from the top, of the Leaning Tower of Pisa in Italy to prove, the same., As g is constant near the earth, all the, equations for the uniformly accelerated, motion of objects become valid with, acceleration a replaced by g (see section 8.5)., The equations are:, v = u + at, (10.10), 1, at2, 2, v2 = u2 + 2as, s = ut +, , (ii) average speed =, , Thus,, (i) its speed on striking the ground, = 5 m s–1, (ii) its average speed during the 0.5 s, = 2.5 m s–1, (iii) height of the ledge from the ground, = 1.25 m., , (10.11), (10.12), , where u and v are the initial and final, velocities and s is the distance covered in, time, t., In applying these equations, we will take, acceleration, a to be positive when it is in the, direction of the velocity, that is, in the, direction of motion. The acceleration, a will, be taken as negative when it opposes the, motion., Example 10.2 A car falls off a ledge and, drops to the ground in 0.5 s. Let, g = 10 m s –2 (for simplifying the, calculations)., (i) What is its speed on striking the, ground?, (ii) What is its average speed during the, 0.5 s?, (iii) How high is the ledge from the, ground?, Solution:, Time, t = ½ second, Initial velocity, u = 0 m s–1, Acceleration due to gravity, g = 10 m s–2, Acceleration of the car, a = + 10 m s–2, (downward), (i) speed, v = at, v = 10 m s–2 × 0.5 s, = 5 m s–1, , Example 10.3 An object is thrown, vertically upwards and rises to a height, of 10 m. Calculate (i) the velocity with, which the object was thrown upwards, and (ii) the time taken by the object to, reach the highest point., Solution:, Distance travelled, s = 10 m, Final velocity, v = 0 m s–1, Acceleration due to gravity, g = 9.8 m s–2, Acceleration of the object, a = –9.8 m s–2, (upward motion), (i), v 2 = u2 + 2a s, 0 = u 2 + 2 × (–9.8 m s–2) × 10 m, –u 2 = –2 × 9.8 × 10 m2 s–2, , (ii), , u = 196 m s-1, u = 14 m s-1, v=u+at, 0 = 14 m s–1 – 9.8 m s–2 × t, t = 1.43 s., , Thus,, (i), Initial velocity, u = 14 m s–1, and, (ii), Time taken, t = 1.43 s., , Q, , 136, , uestions, 1. What do you mean by free fall?, 2. What do you mean by acceleration, due to gravity?, , SCIENCE, , 2019-2020

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10.3 Mass, We have learnt in the previous chapter that the, mass of an object is the measure of its inertia, (section 9.3). We have also learnt that greater, the mass, the greater is the inertia. It remains, the same whether the object is on the earth,, the moon or even in outer space. Thus, the, mass of an object is constant and does not, change from place to place., , 10.4 Weight, We know that the earth attracts every object, with a certain force and this force depends on, the mass (m) of the object and the acceleration, due to the gravity (g). The weight of an object, is the force with which it is attracted towards, the earth., We know that, F = m × a,, (10.13), that is,, (10.14), F = m × g., The force of attraction of the earth on an, object is known as the weight of the object. It, is denoted by W. Substituting the same in Eq., (10.14), we have, W=m×g, (10.15), As the weight of an object is the force with, which it is attracted towards the earth, the SI, unit of weight is the same as that of force, that, is, newton (N). The weight is a force acting, vertically downwards; it has both magnitude, and direction., We have learnt that the value of g is, constant at a given place. Therefore at a given, place, the weight of an object is directly, proportional to the mass, say m, of the object,, that is, W ∝ m. It is due to this reason that at, a given place, we can use the weight of an, object as a measure of its mass. The mass of, an object remains the same everywhere, that, is, on the earth and on any planet whereas its, weight depends on its location because g, depends on location., , 10.4.1 W EIGHT, , OF, , AN, , OBJECT, , attracts the object. In the same way, the weight, of an object on the moon is the force with, which the moon attracts that object. The mass, of the moon is less than that of the earth. Due, to this the moon exerts lesser force of attraction, on objects., Let the mass of an object be m. Let its, weight on the moon be Wm. Let the mass of, the moon be Mm and its radius be Rm., By applying the universal law of, gravitation, the weight of the object on the, moon will be, Wm = G, , Mm × m, , (10.16), , Rm2, , Let the weight of the same object on the, earth be We. The mass of the earth is M and, its radius is R., , Table 10.1, Celestial, body, , Mass (kg), , Radius (m), , Earth, , 5.98 × 1024, , 6.37 × 106, , Moon, , 7.36 × 1022, , 1.74 × 106, , From Eqs. (10.9) and (10.15) we have,, We = G, , M ×m, , (10.17), , R2, , Substituting the values from Table 10.1 in, Eqs. (10.16) and (10.17), we get, Wm = G, , 7.36 × 1022 kg × m, 2, , (1.74 × 10 m ), 6, , Wm = 2.431 × 1010 G × m, 11, , and We = 1.474 × 10 G × m, , (10.18b), , Dividing Eq. (10.18a) by Eq. (10.18b), we get, Wm, We, , =, , 2.431 × 1010, 1.474 × 1011, , Wm, 1, or W = 0.165 ≈ 6, e, Weight of the object on the moon, , ON, , Weight of the object on the earth, , THE MOON, , We have learnt that the weight of an object on, the earth is the force with which the earth, , (10.18a), , (10.19), =, , 1, 6, , Weight of the object on the moon, = (1/6) × its weight on the earth., 137, , GRAVITATION, , 2019-2020

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of the net force in a particular direction (thrust), and the force per unit area (pressure) acting, on the object concerned., Let us try to understand the meanings of, thrust and pressure by considering the, following situations:, Situation 1: You wish to fix a poster on a, bulletin board, as shown in Fig 10.3. To do, this task you will have to press drawing pins, with your thumb. You apply a force on the, surface area of the head of the pin. This force, is directed perpendicular to the surface area, of the board. This force acts on a smaller area, at the tip of the pin., , Example 10.4 Mass of an object is 10 kg., What is its weight on the earth?, Solution:, Mass, m = 10 kg, , Acceleration due to gravity, g = 9.8 m s–2, W=m×g, W = 10 kg × 9.8 m s-2 = 98 N, Thus, the weight of the object is 98 N., Example 10.5 An object weighs 10 N when, measured on the surface of the earth., What would be its weight when, measured on the surface of the moon?, Solution:, We know,, Weight of object on the moon, = (1/6) × its weight on the earth., That is,, Wm =, , We 10, =, N., 6, 6, , = 1.67 N., Thus, the weight of object on the, surface of the moon would be 1.67 N., , Q, , uestions, 1. What are the differences between, the mass of an object and its, weight?, 2. Why is the weight of an object on, the moon, , 1 th, its weight on the, 6, , earth?, , 10.5 Thrust and Pressure, Have you ever wondered why a camel can run, in a desert easily? Why an army tank weighing, more than a thousand tonne rests upon a, continuous chain? Why a truck or a motorbus, has much wider tyres? Why cutting tools have, sharp edges? In order to address these, questions and understand the phenomena, involved, it helps to introduce the concepts, , Fig. 10.3: To fix a poster, drawing pins are pressed, with the thumb perpendicular to the board., , Situation 2: You stand on loose sand. Your, feet go deep into the sand. Now, lie down on, the sand. You will find that your body will not, go that deep in the sand. In both cases the, force exerted on the sand is the weight of your, body., , 138, , SCIENCE, , 2019-2020

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You have learnt that weight is the force, acting vertically downwards. Here the force, is acting perpendicular to the surface of the, sand. The force acting on an object, perpendicular to the surface is called thrust., When you stand on loose sand, the force,, that is, the weight of your body is acting on, an area equal to area of your feet. When you, lie down, the same force acts on an area equal, to the contact area of your whole body, which, is larger than the area of your feet. Thus, the, effects of forces of the same magnitude on, different areas are different. In the above, cases, thrust is the same. But effects are, different. Therefore the effect of thrust, depends on the area on which it acts., The effect of thrust on sand is larger while, standing than while lying. The thrust on unit, area is called pressure. Thus,, Pressure =, , thrust, area, , by the wooden block on the table top if, it is made to lie on the table top with its, sides of dimensions (a) 20 cm × 10 cm, and (b) 40 cm × 20 cm., Solution:, The mass of the wooden block = 5 kg, The dimensions, = 40 cm × 20 cm × 10 cm, Here, the weight of the wooden block, applies a thrust on the table top., That is,, Thrust = F = m × g, = 5 kg × 9.8 m s–2, = 49 N, Area of a side = length × breadth, = 20 cm × 10 cm, = 200 cm2 = 0.02 m2, From Eq. (10.20),, 49 N, , (10.20), , Pressure =, , Substituting the SI unit of thrust and area in, Eq. (10.20), we get the SI unit of pressure as, N/m2 or N m–2., In honour of scientist Blaise Pascal, the, SI unit of pressure is called pascal, denoted, as Pa., Let us consider a numerical example to, understand the effects of thrust acting on, different areas., , 0.02 m 2, , = 2450 N m-2., , When the block lies on its side of, dimensions 40 cm × 20 cm, it exerts, the same thrust., Area= length × breadth, = 40 cm × 20 cm, = 800 cm2 = 0.08 m2, From Eq. (10.20),, 49 N, , Example 10.6 A block of wood is kept on a, tabletop. The mass of wooden block is 5, kg and its dimensions are 40 cm × 20, cm × 10 cm. Find the pressure exerted, , Pressure =, , 0.08 m 2, , = 612.5 N m–2, , The pressure exerted by the side 20 cm, × 10 cm is 2450 N m–2 and by the side, 40 cm × 20 cm is 612.5 N m–2., Thus, the same force acting on a smaller, area exerts a larger pressure, and a smaller, pressure on a larger area. This is the reason, why a nail has a pointed tip, knives have sharp, edges and buildings have wide foundations., , 10.5.1 PRESSURE IN, Fig. 10.4, , FLUIDS, , All liquids and gases are fluids. A solid exerts, pressure on a surface due to its weight., Similarly, fluids have weight, and they also, 139, , GRAVITATION, , 2019-2020

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exert pressure on the base and walls of the, container in which they are enclosed. Pressure, exerted in any confined mass of fluid is, transmitted undiminished in all directions., , 10.5.2 BUOYANCY, Have you ever had a swim in a pool and felt, lighter? Have you ever drawn water from a, well and felt that the bucket of water is heavier, when it is out of the water? Have you ever, wondered why a ship made of iron and steel, does not sink in sea water, but while the same, amount of iron and steel in the form of a sheet, would sink? These questions can be answered, by taking buoyancy in consideration. Let us, understand the meaning of buoyancy by, doing an activity., , Activity _____________ 10.4, •, , •, , •, •, , water on the bottle is greater than its weight., Therefore it rises up when released., To keep the bottle completely immersed,, the upward force on the bottle due to water, must be balanced. This can be achieved by, an externally applied force acting downwards., This force must at least be equal to the, difference between the upward force and the, weight of the bottle., The upward force exerted by the water on, the bottle is known as upthrust or buoyant, force. In fact, all objects experience a force of, buoyancy when they are immersed in a fluid., The magnitude of this buoyant force depends, on the density of the fluid., , 10.5.3 WHY OBJECTS FLOAT OR SINK WHEN, PLACED ON THE SURFACE OF WATER?, Let us do the following activities to arrive at, an answer for the above question., , Take an empty plastic bottle. Close the, mouth of the bottle with an airtight, stopper. Put it in a bucket filled with, water. You see that the bottle floats., Push the bottle into the water. You feel, an upward push. Try to push it further, down. You will find it difficult to push, deeper and deeper. This indicates that, water exerts a force on the bottle in the, upward direction. The upward force, exerted by the water goes on increasing, as the bottle is pushed deeper till it is, completely immersed., Now, release the bottle. It bounces, back to the surface., Does the force due to the gravitational, attraction of the earth act on this, bottle? If so, why doesn’t the bottle stay, immersed in water after it is released?, How can you immerse the bottle in, water?, , Activity _____________ 10.5, •, •, •, , Take a beaker filled with water., Take an iron nail and place it on the, surface of the water., Observe what happens., , The nail sinks. The force due to the, gravitational attraction of the earth on the, iron nail pulls it downwards. There is an, upthrust of water on the nail, which pushes, it upwards. But the downward force acting, on the nail is greater than the upthrust of, water on the nail. So it sinks (Fig. 10.5)., , The force due to the gravitational, attraction of the earth acts on the bottle in, the downward direction. So the bottle is, pulled downwards. But the water exerts an, upward force on the bottle. Thus, the bottle is, pushed upwards. We have learnt that weight, of an object is the force due to gravitational, attraction of the earth. When the bottle is, immersed, the upward force exerted by the, , Fig. 10.5: An iron nail sinks and a cork floats when, placed on the surface of water., , 140, , SCIENCE, , 2019-2020

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Activity _____________ 10.6, •, •, •, •, , Take a beaker filled with water., Take a piece of cork and an iron nail of, equal mass., Place them on the surface of water., Observe what happens., , The cork floats while the nail sinks. This, happens because of the difference in their, densities. The density of a substance is, defined as the mass per unit volume. The, density of cork is less than the density of, water. This means that the upthrust of water, on the cork is greater than the weight of the, cork. So it floats (Fig. 10.5)., The density of an iron nail is more than, the density of water. This means that the, upthrust of water on the iron nail is less than, the weight of the nail. So it sinks., Therefore objects of density less than that, of a liquid float on the liquid. The objects of, density greater than that of a liquid sink in, the liquid., , Q, , uestions, 1. Why is it difficult to hold a school, bag having a strap made of a thin, and strong string?, 2. What do you mean by buoyancy?, 3. Why does an object float or sink, when placed on the surface of, water?, , 10.6 Archimedes’ Principle, Activity _____________ 10.7, •, , •, , •, , •, , Take a piece of stone and tie it to one, end of a rubber string or a spring, balance., Suspend the stone by holding the, balance or the string as shown in, Fig. 10.6 (a)., Note the elongation of the string or, the reading on the spring balance due, to the weight of the stone., Now, slowly dip the stone in the water, in a container as shown in, Fig. 10.6 (b)., , (a), , (b), Fig. 10.6: (a) Observe the elongation of the rubber, string due to the weight of a piece of stone, suspended from it in air. (b) The elongation, decreases as the stone is immersed, in water., , •, , Observe what happens to elongation, of the string or the reading on the, balance., , You will find that the elongation of the string, or the reading of the balance decreases as the, stone is gradually lowered in the water., However, no further change is observed once, the stone gets fully immersed in the water., What do you infer from the decrease in the, extension of the string or the reading of the, spring balance?, We know that the elongation produced in, the string or the spring balance is due to the, weight of the stone. Since the extension, decreases once the stone is lowered in water,, it means that some force acts on the stone in, upward direction. As a result, the net force, on the string decreases and hence the, elongation also decreases. As discussed, earlier, this upward force exerted by water is, known as the force of buoyancy., What is the magnitude of the buoyant, force experienced by a body? Is it the same, in all fluids for a given body? Do all bodies, in a given fluid experience the same buoyant, force? The answer to these questions is, 141, , GRAVITATION, , 2019-2020

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contained in Archimedes’ principle, stated as, follows:, When a body is immersed fully or partially, in a fluid, it experiences an upward force that, is equal to the weight of the fluid displaced, by it., Now, can you explain why a further, decrease in the elongation of the string was, not observed in activity 10.7, as the stone was, fully immersed in water?, Archimedes was a Greek, scientist. He discovered the, principle, subsequently, named after him, after, noticing that the water in a, bathtub overflowed when he, stepped into it. He ran, through the streets shouting, Archimedes, “Eureka!”, which means “I, have got it”. This knowledge helped him to, determine the purity of the gold in the crown, made for the king., His work in the field of Geometry and, , 10.7 Relative Density, As you know, the density of a substance is, defined as mass of a unit volume. The unit of, density is kilogram per metre cube (kg m–3)., The density of a given substance, under, specified conditions, remains the same., Therefore the density of a substance is one, of its characteristic properties. It is different, for different substances. For example, the, density of gold is 19300 kg m-3 while that of, water is 1000 kg m-3. The density of a given, sample of a substance can help us to, determine its purity., It is often convenient to express density, of a substance in comparison with that of, water. The relative density of a substance is, the ratio of its density to that of water:, Relative density =, , Density of a substance, Density of water, , Mechanics made him famous. His, understanding of levers, pulleys, wheelsand-axle helped the Greek army in its war, with Roman army., , Since the relative density is a ratio of, similar quantities, it has no unit., , Archimedes’ principle has many, applications. It is used in designing ships and, submarines. Lactometers, which are used to, determine the purity of a sample of milk and, hydrometers used for determining density of, liquids, are based on this principle., , Example 10.7 Relative density of silver is, 10.8. The density of water is 103 kg m–3., What is the density of silver in SI unit?, , Q, , Solution:, Relative density of silver = 10.8, Relative density, , uestions, , Density of silver, , 1. You find your mass to be 42 kg, on a weighing machine. Is your, mass more or less than 42 kg?, 2. You have a bag of cotton and an, iron bar, each indicating a mass, of 100 kg when measured on a, weighing machine. In reality, one, is heavier than other. Can you, say which one is heavier, and why?, , =, , Density of water, , Density of silver, = Relative density of silver, × density of water, = 10.8 × 103 kg m–3., , 142, , SCIENCE, , 2019-2020

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What, you have, learnt, •, , •, •, , •, •, •, •, •, , The law of gravitation states that the force of attraction between, any two objects is proportional to the product of their masses, and inversely proportional to the square of the distance between, them. The law applies to objects anywhere in the universe., Such a law is said to be universal., Gravitation is a weak force unless large masses are involved., The force of gravity decreases with altitude. It also varies, on the surface of the earth, decreasing from poles to the, equator., The weight of a body is the force with which the earth, attracts it., The weight is equal to the product of mass and acceleration, due to gravity., The weight may vary from place to place but the mass stays, constant., All objects experience a force of buoyancy when they are, immersed in a fluid., Objects having density less than that of the liquid in which, they are immersed, float on the surface of the liquid. If the, density of the object is more than the density of the liquid in, which it is immersed then it sinks in the liquid., , Exercises, 1. How does the force of gravitation between two objects change, when the distance between them is reduced to half ?, 2. Gravitational force acts on all objects in proportion to their, masses. Why then, a heavy object does not fall faster than a, light object?, 3. What is the magnitude of the gravitational force between the, earth and a 1 kg object on its surface? (Mass of the earth is, 6 × 1024 kg and radius of the earth is 6.4 × 106 m.), 4. The earth and the moon are attracted to each other by, gravitational force. Does the earth attract the moon with a, force that is greater or smaller or the same as the force, with which the moon attracts the earth? Why?, 5. If the moon attracts the earth, why does the earth not move, towards the moon?, 143, , GRAVITATION, , 2019-2020

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6. What happens to the force between two objects, if, (i) the mass of one object is doubled?, (ii) the distance between the objects is doubled and tripled?, (iii) the masses of both objects are doubled?, 7. What is the importance of universal law of gravitation?, 8. What is the acceleration of free fall?, 9. What do we call the gravitational force between the earth and, an object?, 10. Amit buys few grams of gold at the poles as per the instruction, of one of his friends. He hands over the same when he meets, him at the equator. Will the friend agree with the weight of gold, bought? If not, why? [Hint: The value of g is greater at the, poles than at the equator.], 11. Why will a sheet of paper fall slower than one that is crumpled, into a ball?, 12. Gravitational force on the surface of the moon is only, , 13., , 14., 15., , 16., , 17., , 18., , 1, as, 6, , strong as gravitational force on the earth. What is the weight, in newtons of a 10 kg object on the moon and on the earth?, A ball is thrown vertically upwards with a velocity of 49 m/s., Calculate, (i) the maximum height to which it rises,, (ii) the total time it takes to return to the surface of the earth., A stone is released from the top of a tower of height 19.6 m., Calculate its final velocity just before touching the ground., A stone is thrown vertically upward with an initial velocity of, 40 m/s. Taking g = 10 m/s2, find the maximum height reached, by the stone. What is the net displacement and the total, distance covered by the stone?, Calculate the force of gravitation between the earth and the, Sun, given that the mass of the earth = 6 × 1024 kg and of the, Sun = 2 × 1030 kg. The average distance between the two is, 1.5 × 1011 m., A stone is allowed to fall from the top of a tower 100 m high, and at the same time another stone is projected vertically, upwards from the ground with a velocity of 25 m/s. Calculate, when and where the two stones will meet., A ball thrown up vertically returns to the thrower after 6 s., Find, (a) the velocity with which it was thrown up,, (b) the maximum height it reaches, and, (c) its position after 4 s., , 144, , SCIENCE, , 2019-2020

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19. In what direction does the buoyant force on an object, immersed in a liquid act?, 20. Why does a block of plastic released under water come up to, the surface of water?, 21. The volume of 50 g of a substance is 20 cm3. If the density of, water is 1 g cm–3, will the substance float or sink?, 22. The volume of a 500 g sealed packet is 350 cm3. Will the packet, float or sink in water if the density of water is 1 g cm–3? What, will be the mass of the water displaced by this packet?, , 145, , GRAVITATION, , 2019-2020

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Chapter 11, WORK, In the previous few chapters we have talked, about ways of describing the motion of, objects, the cause of motion and gravitation., Another concept that helps us understand and, interpret many natural phenomena is ‘work’., Closely related to work are energy and power., In this chapter we shall study these concepts., All living beings need food. Living beings, have to perform several basic activities to, survive. We call such activities ‘life processes’., The energy for these processes comes from, food. We need energy for other activities like, playing, singing, reading, writing, thinking,, jumping, cycling and running. Activities that, are strenuous require more energy., Animals too get engaged in activities. For, example, they may jump and run. They have, to fight, move away from enemies, find food, or find a safe place to live. Also, we engage, some animals to lift weights, carry loads, pull, carts or plough fields. All such activities, require energy., Think of machines. List the machines that, you have come across. What do they need for, their working? Why do some engines require, fuel like petrol and diesel? Why do living, beings and machines need energy?, , 11.1 Work, What is work? There is a difference in the, way we use the term ‘work’ in day-to-day life, and the way we use it in science. To make, this point clear let us consider a few examples., , 11.1.1 NOT, , ‘WORK’, WORKING HARD!, MUCH, , IN SPITE OF, , AND, , ENERGY, , draws diagrams, organises her thoughts,, collects question papers, attends classes,, discusses problems with her friends, and, performs experiments. She expends a lot of, energy on these activities. In common, parlance, she is ‘working hard’. All this ‘hard, work’ may involve very little ‘work’ if we go by, the scientific definition of work., You are working hard to push a huge rock., Let us say the rock does not move despite all, the effort. You get completely exhausted., However, you have not done any work on the, rock as there is no displacement of the rock., You stand still for a few minutes with a, heavy load on your head. You get tired. You, have exerted yourself and have spent quite a, bit of your energy. Are you doing work on the, load? The way we understand the term ‘work’, in science, work is not done., You climb up the steps of a staircase and, reach the second floor of a building just to, see the landscape from there. You may even, climb up a tall tree. If we apply the scientific, definition, these activities involve a lot of work., In day-to-day life, we consider any useful, physical or mental labour as work. Activities, like playing in a field, talking with friends,, humming a tune, watching a movie, attending, a function are sometimes not considered to, be work. What constitutes ‘work’ depends, on the way we define it. We use and define, the term work differently in science. To, understand this let us do the following, activities:, , Activity _____________ 11.1, •, , Kamali is preparing for examinations. She, spends lot of time in studies. She reads books,, , 2019-2020, , We have discussed in the above, paragraphs a number of activities, which we normally consider to be work

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Activity _____________ 11.3, , in day-to-day life. For each of these, activities, ask the following questions, and answer them:, (i) What is the work being done on?, (ii) What is happening to the object?, (iii) Who (what) is doing the work?, , •, , •, , 11.1.2 SCIENTIFIC CONCEPTION OF WORK, To understand the way we view work and, define work from the point of view of science,, let us consider some situations:, Push a pebble lying on a surface. The, pebble moves through a distance. You exerted, a force on the pebble and the pebble got, displaced. In this situation work is done., A girl pulls a trolley and the trolley moves, through a distance. The girl has exerted a, force on the trolley and it is displaced., Therefore, work is done., Lift a book through a height. To do this, you must apply a force. The book rises up., There is a force applied on the book and the, book has moved. Hence, work is done., A closer look at the above situations, reveals that two conditions need to be, satisfied for work to be done: (i) a force should, act on an object, and (ii) the object must be, displaced., If any one of the above conditions does, not exist, work is not done. This is the way, we view work in science., A bullock is pulling a cart. The cart, moves. There is a force on the cart and the, cart has moved. Do you think that work is, done in this situation?, , •, •, , Think of situations when the object is, not displaced in spite of a force acting, on it., Also think of situations when an object, gets displaced in the absence of a force, acting on it., List all the situations that you can, think of for each., Discuss with your friends whether, work is done in these situations., , 11.1.3 WORK DONE BY A CONSTANT FORCE, How is work defined in science? To, understand this, we shall first consider the, case when the force is acting in the direction, of displacement., Let a constant force, F act on an object., Let the object be displaced through a, distance, s in the direction of the force (Fig., 11.1). Let W be the work done. We define work, to be equal to the product of the force and, displacement., Work done = force × displacement, W = Fs, (11.1), , Activity _____________ 11.2, •, •, •, •, •, •, •, , WORK, , Think of some situations from your, daily life involving work., List them., Discuss with your friends whether, work is being done in each situation., Try to reason out your response., If work is done, which is the force acting, on the object?, What is the object on which the work, is done?, What happens to the object on which, work is done?, AND, , Fig. 11.1, , Thus, work done by a force acting on an, object is equal to the magnitude of the force, multiplied by the distance moved in the, direction of the force. Work has only, magnitude and no direction., In Eq. (11.1), if F = 1 N and s = 1 m then, the work done by the force will be 1 N m., Here the unit of work is newton metre (N m), or joule (J). Thus 1 J is the amount of work, 147, , ENERGY, , 2019-2020

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done on an object when a force of 1 N, displaces it by 1 m along the line of action of, the force., Look at Eq. (11.1) carefully. What is the, work done when the force on the object is, zero? What would be the work done when, the displacement of the object is zero? Refer, to the conditions that are to be satisfied to, say that work is done., , Consider a situation in which an object is, moving with a uniform velocity along a, particular direction. Now a retarding force, F,, is applied in the opposite direction. That is,, the angle between the two directions is 180º., Let the object stop after a displacement s. In, such a situation, the work done by the force,, F is taken as negative and denoted by the, minus sign. The work done by the force is, F × (–s) or (–F × s)., It is clear from the above discussion that, the work done by a force can be either positive, or negative. To understand this, let us do the, following activity:, , Example 11.1 A force of 5 N is acting on an, object. The object is displaced through, 2 m in the direction of the force (Fig., 11.2). If the force acts on the object all, through the displacement, then work, done is 5 N × 2 m =10 N m or 10 J., , Fig. 11.2, , Q, , Fig. 11.4, , Activity _____________ 11.4, , uestion, 1. A force of 7 N acts on an object., The displacement is, say 8 m, in, the direction of the force, (Fig. 11.3). Let us take it that the, force acts on the object through, the displacement. What is the, work done in this case?, , •, , •, •, •, , Lift an object up. Work is done by the, force exerted by you on the object. The, object moves upwards. The force you, exerted is in the direction of, displacement. However, there is the, force of gravity acting on the object., Which one of these forces is doing, positive work?, Which one is doing negative work?, Give reasons., , Work done is negative when the force acts, opposite to the direction of displacement., Work done is positive when the force is in the, direction of displacement., , Fig. 11.3, , Consider another situation in which the, force and the displacement are in the same, direction: a baby pulling a toy car parallel to, the ground, as shown in Fig. 11.4. The baby, has exerted a force in the direction of, displacement of the car. In this situation, the, work done will be equal to the product of the, force and displacement. In such situations,, the work done by the force is taken as positive., , Example 11.2 A porter lifts a luggage of 15, kg from the ground and puts it on his, head 1.5 m above the ground. Calculate, the work done by him on the luggage., Solution:, , 148, , Mass of luggage, m = 15 kg and, displacement, s = 1.5 m., SCIENCE, , 2019-2020

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Work done, W = F × s = mg × s, = 15 kg × 10 m s-2 × 1.5 m, = 225 kg m s-2 m, = 225 N m = 225 J, Work done is 225 J., , Q, , uestions, 1. When do we say that work is, done?, 2. Write an expression for the work, done when a force is acting on, an object in the direction of its, displacement., 3. Define 1 J of work., 4. A pair of bullocks exerts a force, of 140 N on a plough. The field, being ploughed is 15 m long., How much work is done in, ploughing the length of the field?, , 11.2 Energy, Life is impossible without energy. The demand, for energy is ever increasing. Where do we, get energy from? The Sun is the biggest, natural source of energy to us. Many of our, energy sources are derived from the Sun. We, can also get energy from the nuclei of atoms,, the interior of the earth, and the tides. Can, you think of other sources of energy?, , Activity _____________ 11.5, •, , •, •, , A few sources of energy are listed above., There are many other sources of, energy. List them., Discuss in small groups how certain, sources of energy are due to the Sun., Are there sources of energy which are, not due to the Sun?, , The word energy is very often used in our, daily life, but in science we give it a definite, and precise meaning. Let us consider the, following examples: when a fast moving, cricket ball hits a stationary wicket, the wicket, is thrown away. Similarly, an object when, raised to a certain height gets the capability, to do work. You must have seen that when a, WORK, , AND, , raised hammer falls on a nail placed on a, piece of wood, it drives the nail into the wood., We have also observed children winding a, toy (such as a toy car) and when the toy is, placed on the floor, it starts moving. When a, balloon is filled with air and we press it we, notice a change in its shape. As long as we, press it gently, it can come back to its original, shape when the force is withdrawn. However,, if we press the balloon hard, it can even, explode producing a blasting sound. In all, these examples, the objects acquire, through, different means, the capability of doing work., An object having a capability to do work is, said to possess energy. The object which does, the work loses energy and the object on which, the work is done gains energy., How does an object with energy do work?, An object that possesses energy can exert a, force on another object. When this happens,, energy is transferred from the former to the, latter. The second object may move as it, receives energy and therefore do some work., Thus, the first object had a capacity to do, work. This implies that any object that, possesses energy can do work., The energy possessed by an object is thus, measured in terms of its capacity of doing, work. The unit of energy is, therefore, the same, as that of work, that is, joule (J). 1 J is the, energy required to do 1 joule of work., Sometimes a larger unit of energy called kilo, joule (kJ) is used. 1 kJ equals 1000 J., , 11.2.1 FORMS OF ENERGY, Luckily the world we live in provides energy, in many different forms. The various forms, include mechanical energy (potential energy, + kinetic energy), heat energy, chemical, energy, electrical energy and light energy., Think it over !, How do you know that some entity is a, form of energy? Discuss with your friends, and teachers., , 149, , ENERGY, , 2019-2020

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James Prescott, Joule was an, outstanding, British physicist., He is best known, for his research in, electricity and, thermodynamics., Amongst other, things,, he, formulated a law, James Prescott Joule for the heating, effect of electric, (1818 – 1889), current. He also, verified experimentally the law of, conservation of energy and discovered, the value of the mechanical equivalent, of heat. The unit of energy and work, called joule, is named after him., , 11.2.2 KINETIC ENERGY, , Fig. 11.5, , •, •, , •, , •, •, , Activity _____________ 11.6, •, , •, •, •, , •, •, •, •, •, , Take a heavy ball. Drop it on a thick, bed of sand. A wet bed of sand would, be better. Drop the ball on the sand, bed from height of about 25 cm. The, ball creates a depression., Repeat this activity from heights of, 50 cm, 1m and 1.5 m., Ensure that all the depressions are, distinctly visible., Mark the depressions to indicate the, height fr om which the ball was, dropped., Compare their depths., Which one of them is deepest?, Which one is shallowest? Why?, What has caused the ball to make a, deeper dent?, Discuss and analyse., , Activity _____________ 11.7, •, •, , •, , Set up the apparatus as shown in, Fig. 11.5., Place a wooden block of known mass, in front of the trolley at a convenient, fixed distance., Place a known mass on the pan so, that the trolley starts moving., , •, •, , The trolley moves forward and hits the, wooden block., Fix a stop on the table in such a, manner that the trolley stops after, hitting the block. The block gets, displaced., Note down the displacement of the, block. This means work is done on the, block by the trolley as the block has, gained energy., From where does this energy come?, Repeat this activity by increasing the, mass on the pan. In which case is the, displacement more?, In which case is the work done more?, In this activity, the moving trolley does, work and hence it possesses energy., , A moving object can do work. An object, moving faster can do more work than an, identical object moving relatively slow. A, moving bullet, blowing wind, a rotating wheel,, a speeding stone can do work. How does a, bullet pierce the target? How does the wind, move the blades of a windmill? Objects in, motion possess energy. We call this energy, kinetic energy., A falling coconut, a speeding car, a rolling, stone, a flying aircraft, flowing water, blowing, wind, a running athlete etc. possess kinetic, energy. In short, kinetic energy is the energy, possessed by an object due to its motion. The, kinetic energy of an object increases with its, speed., How much energy is possessed by a, moving body by virtue of its motion? By, definition, we say that the kinetic energy of a, body moving with a certain velocity is equal to, the work done on it to make it acquire that, velocity., , 150, , SCIENCE, , 2019-2020

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Let us now express the kinetic energy of, an object in the form of an equation. Consider, an object of mass, m moving with a uniform, velocity, u. Let it now be displaced through a, distance s when a constant force, F acts on it, in the direction of its displacement. From, Eq. (11.1), the work done, W is F s. The work, done on the object will cause a change in its, velocity. Let its velocity change from u to v., Let a be the acceleration produced., In section 8.5, we studied three equations, of motion. The relation connecting the initial, velocity (u) and final velocity (v) of an object, moving with a uniform acceleration a, and, the displacement, s is, v2 – u2 = 2a s, (8.7), This gives, s=, , v2 – u 2, 2a, , (11.2), , From section 9.4, we know F = m a. Thus,, using (Eq. 11.2) in Eq. (11.1), we can write, the work done by the force, F as, v2 - u 2, W =ma ×, 2a, , (, , Mass of the object, m = 15 kg, velocity, of the object, v = 4 m s–1., From Eq. (11.5),, Ek =, , 1, × 15 kg × 4 m s–1 × 4 m s–1, 2, = 120 J, The kinetic energy of the object is 120 J., , Example 11.4 What is the work to be done, to increase the velocity of a car from, 30 km h–1 to 60 km h–1 if the mass of, the car is 1500 kg?, Solution:, Mass of the car, m =1500 kg,, initial velocity of car, u = 30 km h–1, 30 × 1000 m, 60 × 60 s, , = 25/3 m s–1., Similarly, the final velocity of the car,, , ), , (11.3), , If the object is starting from its stationary, position, that is, u = 0, then, 1, W = m v2, 2, , (11.4), , v = 60 km h–1, = 50/3 m s–1., , Therefore, the initial kinetic energy of, the car,, Eki =, , It is clear that the work done is equal to the, change in the kinetic energy of an object., If u = 0, the work done will be, , 1, m v2, 2, , (11.5), , Example 11.3 An object of mass 15 kg is, moving with a uniform velocity of, 4, m s –1 . What is the kinetic energy, possessed by the object?, WORK, , AND, , 1, m u2, 2, , 1, × 1500 kg × (25/3 m s–1)2, 2, = 156250/3 J., , =, , 1, m v2 ., 2, , Thus, the kinetic energy possessed by an, object of mass, m and moving with a uniform, velocity, v is, Ek =, , 1, m v2, 2, , =, , =, , or, 1, W = m v2 – u 2, 2, , Solution:, , The final kinetic energy of the car,, Ekf =, , 1, × 1500 kg × (50/3 m s–1)2, 2, = 625000/3 J., , Thus, the work done =, kinetic energy, = Ekf – Eki, , Change in, , = 156250 J., 151, , ENERGY, , 2019-2020

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Q, , uestions, 1. What is the kinetic energy of an, object?, 2. Write an expression for the kinetic, energy of an object., 3. The kinetic energy of an object of, mass, m moving with a velocity, of 5 m s–1 is 25 J. What will be its, kinetic energy when its velocity, is doubled? What will be its, kinetic energy when its velocity, is increased three times?, , 11.2.3 POTENTIAL ENERGY, , Activity _____________ 11.8, •, •, •, •, •, , •, , Take a rubber band., Hold it at one end and pull from the, other. The band stretches., Release the band at one of the ends., What happens?, The band will tend to regain its original, length. Obviously the band had, acquir ed energy in its stretched, position., How did it acquire energy when, stretched?, , Activity _____________ 11.9, •, •, , Take a slinky as shown below., Ask a friend to hold one of its ends., You hold the other end and move away, from your friend. Now you release the, slinky., , Activity ___________ 11.11, •, , •, , •, , In the above situations, the energy gets, stored due to the work done on the object., The energy transferred to an object is stored, as potential energy if it is not used to cause a, change in the velocity or speed of the object., You transfer energy when you stretch a, rubber band. The energy transferred to the, band is its potential energy. You do work while, winding the key of a toy car. The energy, transferred to the spring inside is stored as, potential energy. The potential energy, possessed by the object is the energy present, in it by virtue of its position or configuration., , Activity ____________11.12, •, •, , •, •, , •, •, •, •, , Lift an object through a certain, height. The object can now do work., It begins to fall when released., This implies that it has acquired some, energy. If raised to a greater height it, can do more work and hence possesses, more energy., From where did it get the energy? Think, and discuss., , What happened?, How did the slinky acquire energy when, stretched?, Would the slinky acquire energy when, it is compressed?, , Take a bamboo stick and make a bow, as shown in Fig. 11.6., Place an arrow made of a light stick on, it with one end supported by the, stretched string., Now stretch the string and release the, arrow., Notice the arrow flying off the bow., Notice the change in the shape of the, bow., The potential energy stored in the bow, due to the change of shape is thus used, in the for m of kinetic energy in, throwing off the arrow., , Activity ____________11.10, •, •, •, •, •, •, , Take a toy car. Wind it using its key., Place the car on the ground., Did it move?, From where did it acquire energy?, Does the energy acquired depend on, the number of windings?, How can you test this?, , Fig.11.6: An arrow and the stretched string, on the bow., , 152, , SCIENCE, , 2019-2020

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AT A HEIGHT, , An object increases its energy when raised, through a height. This is because work is, done on it against gravity while it is being, raised. The energy present in such an object, is the gravitational potential energy., The gravitational potential energy of an, object at a point above the ground is defined, as the work done in raising it from the ground, to that point against gravity., It is easy to arrive at an expression for, the gravitational potential energy of an object, at a height., , More to know, , 11.2.4 POTENTIAL ENERGY OF AN OBJECT, , The potential energy of an object at, a height depends on the ground level, or the zero level you choose. An, object in a given position can have a, certain potential energy with respect, to one level and a different value of, potential energy with respect to, another level., , It is useful to note that the work done by, gravity depends on the difference in vertical, heights of the initial and final positions of, the object and not on the path along which, the object is moved. Fig. 11.8 shows a case, where a block is raised from position A to B, by taking two different paths. Let the height, AB = h. In both the situations the work done, on the object is mgh., , Fig. 11.7, Fig. 11.8, , Consider an object of mass, m. Let it be, raised through a height, h from the ground., A force is required to do this. The minimum, force required to raise the object is equal to, the weight of the object, mg. The object gains, energy equal to the work done on it. Let the, work done on the object against gravity be, W. That is,, work done, W = force × displacement, = mg × h, = mgh, Since work done on the object is equal to, mgh, an energy equal to mgh units is gained, by the object. This is the potential energy (EP), of the object., Ep = mgh, (11.7), WORK, , AND, , Example 11.5 Find the energy possessed, by an object of mass 10 kg when it is at, a height of 6 m above the ground. Given,, g = 9.8 m s–2., Solution:, Mass of the object, m = 10 kg,, displacement (height), h = 6 m, and, acceleration due to gravity, g = 9.8 m s–2., From Eq. (11.6),, Potential energy = mgh, = 10 kg × 9.8 m s–2 × 6 m, = 588 J., The potential energy is 588 J., 153, , ENERGY, , 2019-2020

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11.2.6 LAW OF CONSERVATION OF ENERGY, , Example 11.6 An object of mass 12 kg is, at a certain height above the ground., If the potential energy of the object is, 480 J, find the height at which the, object is with respect to the ground., Given, g = 10 m s–2., Solution:, Mass of the object, m = 12 kg,, potential energy, Ep = 480 J., Ep = mgh, 480 J = 12 kg × 10 m s–2 × h, , 480 J, h = 120 kg m s –2 = 4 m., The object is at the height of 4 m., , 11.2.5 A R E, , VARIOUS ENERGY FORMS, , INTERCONVERTIBLE?, , Can we convert energy from one form to, another? We find in nature a number of, instances of conversion of energy from one, form to another., , Activity ____________11.13, •, •, •, , Sit in small groups., Discuss the various ways of energy, conversion in nature., Discuss following questions in your, group:, (a) How do green plants produce food?, (b) Where do they get their energy from?, (c) Why does the air move from place, to place?, (d) How are fuels, such as coal and, petroleum formed?, (e) What kinds of energy conversions, sustain the water cycle?, , Activity ___________ 11.14, •, , •, •, , Many of the human activities and the, gadgets we use involve conversion of, energy from one form to another., Make a list of such activities and, gadgets., Identify in each activity/gadget the, kind of energy conversion that takes, place., , In activities 11.13 and 11.14, we learnt that, the form of energy can be changed from one, form to another. What happens to the total, energy of a system during or after the process?, Whenever energy gets transformed, the total, energy remains unchanged. This is the law of, conservation of energy. According to this law,, energy can only be converted from one form, to another; it can neither be created or, destroyed. The total energy before and after, the transformation remains the same. The law, of conservation of energy is valid, in, all situations and for all kinds of, transformations., Consider a simple example. Let an object, of mass, m be made to fall freely from a height,, h. At the start, the potential energy is mgh and, kinetic energy is zero. Why is the kinetic, energy zero? It is zero because its velocity is, zero. The total energy of the object is thus mgh., As it falls, its potential energy will change into, kinetic energy. If v is the velocity of the object, at a given instant, the kinetic energy would be, ½mv2. As the fall of the object continues, the, potential energy would decrease while the, kinetic energy would increase. When the object, is about to reach the ground, h = 0 and v will, be the highest. Therefore, the kinetic energy, would be the largest and potential energy the, least. However, the sum of the potential energy, and kinetic energy of the object would be the, same at all points. That is,, potential energy + kinetic energy = constant, or, mgh +, , 1, mv 2= constant., 2, , (11.7), , The sum of kinetic energy and potential energy, of an object is its total mechanical energy., We find that during the free fall of the object,, the decrease in potential energy, at any point, in its path, appears as an equal amount of, increase in kinetic energy. (Here the effect of, air resistance on the motion of the object has, been ignored.) There is thus a continual, transformation of gravitational potential, energy into kinetic energy., , 154, , SCIENCE, , 2019-2020

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Activity ___________ 11.15, An object of mass 20 kg is dropped, from a height of 4 m. Fill in the blanks, in the following table by computing, the potential energy and kinetic, energy in each case., , •, , Height at, Potential, Kinetic, Ep + Ek, which object energy, energy, is located (Ep= mgh) (Ek = mv2/2), m, , J, , J, , J, , 4, 3, , A stronger person may do certain work in, relatively less time. A more powerful vehicle, would complete a journey in a shorter time, than a less powerful one. We talk of the power, of machines like motorbikes and motorcars., The speed with which these vehicles change, energy or do work is a basis for their, classification. Power measures the speed of, work done, that is, how fast or slow work is, done. Power is defined as the rate of doing, work or the rate of transfer of energy. If an, agent does a work W in time t, then power is, given by:, Power = work/time, , 2, 1, , or, , Just above, the ground, •, , For simplifying the calculations, take, the value of g as 10 m s–2., , Think it over !, What would have happened if nature had, not allowed the transformation of energy?, There is a view that life could not have, been possible without transformation of, energy. Do you agree with this?, , 11.3 Rate of Doing Work, Do all of us work at the same rate? Do, machines consume or transfer energy at the, same rate? Agents that transfer energy do, work at different rates. Let us understand this, from the following activity:, , •, •, , •, , WORK, , Consider two children, say A and B., Let us say they weigh the same. Both, start climbing up a rope separately., Both reach a height of 8 m. Let us say, A takes 15 s while B takes 20 s to, accomplish the task., What is the work done by each?, The work done is the same. However,, A has taken less time than B to do, the work., Who has done more work in a given, time, say in 1 s?, AND, , W, t, , (11.8), , The unit of power is watt [in honour of, James Watt (1736 – 1819)] having the symbol, W. 1 watt is the power of an agent, which, does work at the rate of 1 joule per second., We can also say that power is 1 W when the, rate of consumption of energy is 1 J s–1., 1 watt = 1 joule/second or 1 W = 1 J s–1., We express larger rates of energy transfer in, kilowatts (kW)., 1 kilowatt = 1000 watts, 1 kW, = 1000 W, 1 kW, = 1000 J s–1., The power of an agent may vary with time., This means that the agent may be doing work, at different rates at different intervals of time., Therefore the concept of average power is, useful. We obtain average power by dividing, the total energy consumed by the total time, taken., , Activity ___________ 11.16, •, , P=, , Example 11.7 Two girls, each of weight 400, N climb up a rope through a height of 8, m. We name one of the girls A and the, other B. Girl A takes 20 s while B takes, 50 s to accomplish this task. What is the, power expended by each girl?, Solution:, (i) Power expended by girl A:, Weight of the girl, mg = 400 N, Displacement (height), h = 8 m, 155, , ENERGY, , 2019-2020

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Time taken, t = 20 s, From Eq. (11.8),, Power, P = Work done/time taken, =, , mgh, t, , 400 N × 8 m, , =, , 11.3.1 COMMERCIAL UNIT OF ENERGY, , (ii) Power expended by girl B:, Weight of the girl, mg = 400 N, Displacement (height), h = 8 m, Time taken, t = 50 s, Power, P =, , mgh, t, , 400 N × 8 m, , =, , 50 s, , = 64 W., , Power expended by girl A is 160 W., Power expended by girl B is 64 W., , Example 11.8 A boy of mass 50 kg runs, up a staircase of 45 steps in 9 s. If the, height of each step is 15 cm, find his, power. Take g = 10 m s–2., Solution:, Weight of the boy,, mg = 50 kg × 10 m s–2 = 500 N, Height of the staircase,, h = 45 × 15/100 m = 6.75 m, Time taken to climb, t = 9 s, From Eq. (11.8),, power, P = Work done/time taken, , =, , 1. What is power?, 2. Define 1 watt of power., 3. A lamp consumes 1000 J of, electrical energy in 10 s. What is, its power?, 4. Define average power., , 20s, , = 160 W., , =, , Q, , uestions, , The unit joule is too small and hence is, inconvenient to express large quantities of, energy. We use a bigger unit of energy called, kilowatt hour (kW h)., What is 1 kW h? Let us say we have a, machine that uses 1000 J of energy every, second. If this machine is used continuously, for one hour, it will consume 1 kW h of energy., Thus, 1 kW h is the energy used in one hour, at the rate of 1000 J s–1 (or 1 kW)., 1 kW h = 1 kW ×1 h, = 1000 W × 3600 s, = 3600000 J, 1 kW h = 3.6 × 106 J., The energy used in households, industries, and commercial establishments are usually, expressed in kilowatt hour. For example,, electrical energy used during a month is, expressed in terms of ‘units’. Here, 1 ‘unit’, means 1 kilowatt hour., Example 11.9 An electric bulb of 60 W is, used for 6 h per day. Calculate the ‘units’, of energy consumed in one day by the, bulb., Solution:, Power of electric bulb = 60 W, = 0.06 kW., Time used, t = 6 h, Energy = power × time taken, = 0.06 kW × 6 h, = 0.36 kW h, = 0.36 ‘units’., The energy consumed by the bulb is, 0.36 ‘units’., , mgh, t, 500 N × 6.75 m, 9s, , = 375 W., , Power is 375 W., 156, , SCIENCE, , 2019-2020

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Activity ___________ 11.17, •, , •, •, •, , Take a close look at the electric meter, installed in your house. Observe its, features closely., Take the readings of the meter each, day at 6.30 am and 6.30 pm., Do this activity for about a week., How many ‘units’ are consumed, during day time?, , •, •, •, •, , How many ‘units’ are used during night?, Tabulate your observations., Draw inferences from the data., Compare your observations with, the details given in the monthly, electricity bill (One can also estimate, the electricity to be consumed by, specific appliances by tabulating their, known wattages and hours of, operation)., , What, you have, learnt, •, , Work done on an object is defined as the magnitude of the, force multiplied by the distance moved by the object in the, direction of the applied force. The unit of work is joule:, 1 joule = 1 newton × 1 metre., , •, , Work done on an object by a force would be zero if the, displacement of the object is zero., , •, , An object having capability to do work is said to possess energy., Energy has the same unit as that of work., , •, , An object in motion possesses what is known as the kinetic, energy of the object. An object of mass, m moving with velocity, 1, 2, v has a kinetic energy of 2 mv ., , WORK, , AND, , •, , The energy possessed by a body due to its change in position, or shape is called the potential energy. The gravitational, potential energy of an object of mass, m raised through a height,, h from the earth’s surface is given by m g h., , •, , According to the law of conservation of energy, energy can, only be transformed from one form to another; it can neither, be created nor destroyed. The total energy before and after, the transformation always remains constant., , •, , Energy exists in nature in several forms such as kinetic, energy, potential energy, heat energy, chemical energy etc., The sum of the kinetic and potential energies of an object is, called its mechanical energy., , •, , Power is defined as the rate of doing work. The SI unit of, power is watt. 1 W = 1 J/s., , •, , The energy used in one hour at the rate of 1kW is called 1 kW h., 157, , ENERGY, , 2019-2020

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Exercises, 1. Look at the activities listed below. Reason out whether or, not work is done in the light of your understanding of the, term ‘work’., •, Suma is swimming in a pond., •, A donkey is carrying a load on its back., •, A wind-mill is lifting water from a well., •, A green plant is carrying out photosynthesis., •, An engine is pulling a train., •, Food grains are getting dried in the sun., •, A sailboat is moving due to wind energy., 2. An object thrown at a certain angle to the ground moves in a, curved path and falls back to the ground. The initial and the, final points of the path of the object lie on the same horizontal, line. What is the work done by the force of gravity on the object?, 3. A battery lights a bulb. Describe the energy changes involved, in the process., 4. Certain force acting on a 20 kg mass changes its velocity from, 5 m s–1 to 2 m s–1. Calculate the work done by the force., 5. A mass of 10 kg is at a point A on a table. It is moved to a point, B. If the line joining A and B is horizontal, what is the work, done on the object by the gravitational force? Explain your, answer., 6. The potential energy of a freely falling object decreases, progressively. Does this violate the law of conservation of, energy? Why?, 7. What are the various energy transformations that occur when, you are riding a bicycle?, 8. Does the transfer of energy take place when you push a, huge rock with all your might and fail to move it? Where is, the energy you spend going?, 9. A certain household has consumed 250 units of energy during, a month. How much energy is this in joules?, 10. An object of mass 40 kg is raised to a height of 5 m above the, ground. What is its potential energy? If the object is allowed to, fall, find its kinetic energy when it is half-way down., 11. What is the work done by the force of gravity on a satellite, moving round the earth? Justify your answer., 12. Can there be displacement of an object in the absence of any, force acting on it? Think. Discuss this question with your, friends and teacher., 158, , SCIENCE, , 2019-2020

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13. A person holds a bundle of hay over his head for 30 minutes, and gets tired. Has he done some work or not? Justify your, answer., 14. An electric heater is rated 1500 W. How much energy does it, use in 10 hours?, 15. Illustrate the law of conservation of energy by discussing the, energy changes which occur when we draw a pendulum, bob to one side and allow it to oscillate. Why does the bob, eventually come to rest? What happens to its energy, eventually? Is it a violation of the law of conservation of, energy?, 16. An object of mass, m is moving with a constant velocity, v., How much work should be done on the object in order to bring, the object to rest?, 17. Calculate the work required to be done to stop a car of 1500 kg, moving at a velocity of 60 km/h?, 18. In each of the following a force, F is acting on an object of, mass, m. The direction of displacement is from west to east, shown by the longer arrow. Observe the diagrams carefully, and state whether the work done by the force is negative,, positive or zero., , 19. Soni says that the acceleration in an object could be zero, even when several forces are acting on it. Do you agree with, her? Why?, 20. Find the energy in kW h consumed in 10 hours by four, devices of power 500 W each., 21. A freely falling object eventually stops on reaching the, ground. What happenes to its kinetic energy?, , WORK, , AND, , 159, , ENERGY, , 2019-2020

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Chapter 12, SOUND, Everyday we hear sounds from various, sources like humans, birds, bells, machines,, vehicles, televisions, radios etc. Sound is a, form of energy which produces a sensation, of hearing in our ears. There are also other, forms of energy like mechanical energy, light, energy, etc. We have talked about mechanical, energy in the previous chapters. You have, been taught about conservation of energy,, which states that we can neither create nor, destroy energy. We can just change it from, one form to another. When you clap, a sound, is produced. Can you produce sound without, utilising your energy? Which form of energy, did you use to produce sound? In this, chapter we are going to learn how sound is, produced and how it is transmitted through, a medium and received by our ears., , 12.1 Production of Sound, , Fig. 12.1: Vibrating tuning fork just touching the, suspended table tennis ball., , Activity _____________ 12.2, •, , Activity _____________ 12.1, •, , •, •, , •, , •, , Take a tuning fork and set it vibrating, by striking its prong on a rubber pad., Bring it near your ear., Do you hear any sound?, Touch one of the prongs of the vibrating, tuning fork with your finger and share, your experience with your friends., Now, suspend a table tennis ball or a, small plastic ball by a thread from a, support [Take a big needle and a, thread, put a knot at one end of the, thread, and then with the help of the, needle pass the thread through the, ball]. Touch the ball gently with the, pr ong of a vibrating tuning, fork (Fig. 12.1)., Observe what happens and discuss, with your friends., , •, , •, •, , Fill water in a beaker or a glass up to, the brim. Gently touch the water surface, with one of the prongs of the vibrating, tuning fork, as shown in Fig. 12.2., Next dip the prongs of the vibrating, tuning fork in water, as shown in Fig., 12.3., Observe what happens in both the, cases., Discuss with your friends why this, happens., , Fig. 12.2: One of the prongs of the vibrating tuning, fork touching the water surface., , 2019-2020

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plucked vibrates and produces sound. If you, have never done this, then do it and observe, the vibration of the stretched rubber band., , Activity _____________ 12.3, •, , Fig. 12.3: Both the prongs of the vibrating tuning fork, dipped in water., , From the above activities what do you, conclude? Can you produce sound without, a vibrating object?, In the above activities we have produced, sound by striking the tuning fork. We can, also produce sound by plucking, scratching,, rubbing, blowing or shaking different objects., As per the above activities what do we do to, the objects? We set the objects vibrating and, produce sound. Vibration means a kind of, rapid to and fro motion of an object. The, sound of the human voice is produced due, to vibrations in the vocal cords. When a bird, flaps its wings, do you hear any sound? Think, how the buzzing sound accompanying a bee, is produced. A stretched rubber band when, , Make a list of different types of musical, instruments and discuss with your, friends which part of the instrument, vibrates to produce sound., , 12.2 Propagation of Sound, Sound is produced by vibrating objects. The, matter or substance through which sound, is transmitted is called a medium. It can be, solid, liquid or gas. Sound moves through a, medium from the point of generation to the, listener. When an object vibrates, it sets the, particles of the medium around it vibrating., The particles do not travel all the way from, the vibrating object to the ear. A particle of, the medium in contact with the vibrating, object is first displaced from its equilibrium, position. It then exerts a force on the adjacent, particle. As a result of which the adjacent, particle gets displaced from its position of, rest. After displacing the adjacent particle the, first particle comes back to its original, position. This process continues in the, medium till the sound reaches your ear. The, disturbance created by a source of sound in, , Can sound make a light spot dance?, Take a tin-can. Remove both ends to make it a hollow cylinder. Take a balloon and stretch it, over the can, then wrap a rubber band around the balloon. Take a small piece of mirror. Use, a drop of glue to stick the piece of mirror to the balloon. Allow the light through a slit to fall on, the mirror. After reflection the light spot is seen on the wall, as shown in Fig. 12.4. Talk or, shout directly into the open end of the can and observe the dancing light spot on the wall., Discuss with your friends what makes the light spot dance., , Fig. 12.4: A beam of light from a light source is made to fall on a mirror. The reflected light is falling on the wall., 161, , SOUND, , 2019-2020

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the medium travels through the medium and, not the particles of the medium., A wave is a disturbance that moves, through a medium when the particles of the, medium set neighbouring particles into, motion. They in turn produce similar motion, in others. The particles of the medium do not, move forward themselves, but the, disturbance is carried forward. This is what, happens during propagation of sound in a, medium, hence sound can be visualised as a, wave. Sound waves are characterised by the, motion of particles in the medium and are, called mechanical waves., Air is the most common medium through, which sound travels. When a vibrating object, moves forward, it pushes and compresses the, air in front of it creating a region of high, pressure. This region is called a compression, (C), as shown in Fig. 12.5. This compression, starts to move away from the vibrating object., When the vibrating object moves backwards,, it creates a region of low pressure called, rarefaction (R), as shown in Fig. 12.5. As the, object moves back and forth rapidly, a series, of compressions and rarefactions is created, in the air. These make the sound wave that, propagates through the medium., Compression is the region of high pressure, and rarefaction is the region of low pressure., Pressure is related to the number of particles, of a medium in a given volume. More density, of the particles in the medium gives more, pressure and vice versa. Thus, propagation, of sound can be visualised as propagation of, density variations or pressure variations in, the medium., , Fig. 12.5: A vibrating object creating a series of, compressions (C) and rarefactions (R) in, the medium., , Q, , uestion, 1. How does the sound produced by, a vibrating object in a medium, reach your ear?, , 12.2.1 SOUND NEEDS A MEDIUM TO TRAVEL, Sound is a mechanical wave and needs a, material medium like air, water, steel etc. for, its propagation. It cannot travel through, vacuum, which can be demonstrated by the, following experiment., Take an electric bell and an airtight glass, bell jar. The electric bell is suspended inside, the airtight bell jar. The bell jar is connected, to a vacuum pump, as shown in Fig. 12.6. If, you press the switch you will be able to hear, the bell. Now start the vacuum pump. When, the air in the jar is pumped out gradually,, the sound becomes fainter, although the, same current is passing through the bell., After some time when less air is left inside, the bell jar you will hear a very feeble sound., What will happen if the air is removed, completely? Will you still be able to hear the, sound of the bell?, , Fig. 12.6: Bell jar experiment showing sound cannot, travel in vacuum., , 162, , SCIENCE, , 2019-2020

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Q, , uestions, 1. Explain how sound is produced, by your school bell., 2. Why are sound waves called, mechanical waves?, 3. Suppose you and your friend are, on the moon. Will you be able to, hear any sound produced by, your friend?, , 12.2.2 SOUND WAVES ARE LONGITUDINAL, WAVES, , Activity _____________ 12.4, •, , •, , •, , Take a slinky. Ask your friend to hold, one end. You hold the other end., Now stretch the slinky as shown in, Fig. 12.7 (a). Then give it a sharp push, towards your friend., What do you notice? If you move your, hand pushing and pulling the slinky, alternatively, what will you observe?, If you mark a dot on the slinky, you, will observe that the dot on the slinky, will move back and forth parallel to the, direction of the propagation of the, disturbance., , waves. In these waves the individual particles, of the medium move in a direction parallel to, the direction of propagation of the, disturbance. The particles do not move from, one place to another but they simply oscillate, back and forth about their position of rest., This is exactly how a sound wave propagates,, hence sound waves are longitudinal waves., There is also another type of wave, called, a transverse wave. In a transverse wave, particles do not oscillate along the direction, of wave propagation but oscillate up and down, about their mean position as the wave travels., Thus, a transverse wave is the one in which, the individual particles of the medium move, about their mean positions in a direction, perpendicular to the direction of wave, propagation. When we drop a pebble in a, pond, the waves you see on the water surface, is an example of transverse wave. Light is a, transverse wave but for light, the oscillations, are not of the medium particles or their, pressure or density – it is not a mechanical, wave. You will come to know more about, transverse waves in higher classes., , 12.2.3 CHARACTERISTICS, , OF A SOUND, , WAVE, , We can describe a sound wave by its, • frequency, • amplitude and, • speed., , (a), , (b), Fig. 12.7: Longitudinal wave in a slinky., , The regions where the coils become closer, are called compressions (C) and the regions, where the coils are further apart are called, rarefactions (R). As we already know, sound, propagates in the medium as a series of, compressions and rarefactions. Now, we can, compare the propagation of disturbance in a, slinky with the sound propagation in the, medium. These waves are called longitudinal, , A sound wave in graphic form is shown, in Fig. 12.8(c), which represents how density, and pressure change when the sound wave, moves in the medium. The density as well as, the pressure of the medium at a given time, varies with distance, above and below the, average value of density and pressure., Fig. 12.8(a) and Fig. 12.8(b) represent the, density and pressure variations, respectively,, as a sound wave propagates in the medium., Compressions are the regions where, particles are crowded together and, represented by the upper portion of the curve, in Fig. 12.8(c). The peak represents the region, of, maximum, compression., Thus,, compressions are regions where density as, 163, , SOUND, , 2019-2020

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Fig. 12.8: Sound propagates as density or pressure variations as shown in (a) and (b), (c) represents, graphically the density and pressure variations., , well as pressure is high. Rarefactions are the, regions of low pressure where particles are, spread apart and are represented by the, valley, that is, the lower portion of the curve, in Fig. 12.8(c). A peak is called the crest and a, valley is called the trough of a wave., The distance between two consecutive, compressions (C) or two consecutive, rarefactions (R) is called the wavelength, as, shown in Fig. 12.8(c), The wavelength is, usually represented by λ (Greek letter, lambda). Its SI unit is metre (m)., Heinrich Rudolph Hertz, was born on 22 February, 1857, in, Hamburg,, Germany and educated at, the University of Berlin. He, confirmed J.C. Maxwell’s, electromagnetic theory by, his experiments. He laid the, H. R. Hertz, foundation for future, development of radio, telephone, telegraph, and even television. He also discovered the, , photoelectric effect which was later, explained by Albert Einstein. The SI unit of, frequency was named as hertz in his honour., Frequency tells us how frequently an event, occurs. Suppose you are beating a drum. How, many times you are beating the drum in unit, time is called the frequency of your beating, the drum. We know that when sound is, propagated through a medium, the density, of the medium oscillates between a maximum, value and a minimum value. The change in, density from the maximum value to the, minimum value, then again to the maximum, value, makes one complete oscillation. The, number of such oscillations per unit time is, the frequency of the sound wave. If we can, count the number of the compressions or, rarefactions that cross us per unit time, we, will get the frequency of the sound wave. It is, usually represented by ν (Greek letter, nu). Its, SI unit is hertz (symbol, Hz)., The time taken by two consecutive, compressions or rarefactions to cross a fixed, point is called the time period of the wave. In, , 164, , SCIENCE, , 2019-2020

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other words, we can say that the time taken, for one complete oscillation is called the time, period of the sound wave. It is represented by, the symbol T. Its SI unit is second (s)., Frequency and time period are related as, follows:, 1, ν = ., T, A violin and a flute may both be played, at the same time in an orchestra. Both, sounds travel through the same medium,, that is, air and arrive at our ear at the same, time. Both sounds travel at the same speed, irrespective of the source. But the sounds, we receive are different. This is due to the, different characteristics associated with the, sound. Pitch is one of the characteristics., How the brain interprets the frequency of, an emitted sound is called its pitch. The faster, the vibration of the source, the higher is, the frequency and the higher is the pitch,, as shown in Fig. 12.9. Thus, a high pitch, sound corresponds to more number of, compressions and rarefactions passing a, fixed point per unit time., Objects of different sizes and conditions, vibrate at different frequencies to produce, sounds of different pitch., , The magnitude of the maximum, disturbance in the medium on either side of, the mean value is called the amplitude of the, wave. It is usually represented by the letter A,, as shown in Fig. 12.8(c). For sound its unit, will be that of density or pressure., The loudness or softness of a sound is, determined basically by its amplitude. The, amplitude of the sound wave depends upon, the force with which an object is made to, vibrate. If we strike a table lightly, we hear a, soft sound because we produce a sound wave, of less energy (amplitude). If we hit the table, hard we hear a louder sound. Can you tell, why? A sound wave spreads out from its, source. As it moves away from the source its, amplitude as well as its loudness decreases., Louder sound can travel a larger distance as, it is associated with higher energy. Fig. 12.10, shows the wave shapes of a loud and a soft, sound of the same frequency., , Fig. 12.10: Soft sound has small amplitude and, louder sound has large amplitude., , Fig. 12.9: Low pitch sound has low frequency and, high pitch of sound has high frequency., , The quality or timber of sound is that, characteristic which enables us to distinguish, one sound from another having the same pitch, and loudness. The sound which is more, 165, , SOUND, , 2019-2020

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pleasant is said to be of a rich quality. A sound, of single frequency is called a tone. The sound, which is produced due to a mixture of several, frequencies is called a note and is pleasant to, listen to. Noise is unpleasant to the ear! Music, is pleasant to hear and is of rich quality., , Q, , uestions, 1. Which wave property determines, (a) loudness, (b) pitch?, 2. Guess which sound has a higher, pitch: guitar or car horn?, , The speed of sound is defined as the, distance which a point on a wave, such as a, compression or a rarefaction, travels per unit, time., We know,, speed, v = distance / time, =, , The time taken by the wave to travel a, distance, d of 1.5 km is, , Thus sound will take 2.1 s to travel a, distance of 1.5 km., , Q, , λ, T, , Here λ is the wavelength of the sound wave., It is the distance travelled by the sound wave, in one time period (T) of the wave. Thus,, v=λν, , or, v=λν, That is, speed = wavelength × frequency., The speed of sound remains almost the, same for all frequencies in a given medium, under the same physical conditions., Example 12.1 A sound wave has a, frequency of 2 kHz and wave length, 35 cm. How long will it take to travel, 1.5 km?, Solution:, Given,, Frequency, ν = 2 kHz = 2000 Hz, Wavelength, λ = 35 cm = 0.35 m, We know that speed, v of the wave, = wavelength × frequency, v =λν, = 0.35 m × 2000 Hz = 700 m/s, , uestions, 1. What are wavelength, frequency,, time period and amplitude of a, sound wave?, 2. How are the wavelength and, frequency of a sound wave, related to its speed?, 3. Calculate the wavelength of a, sound wave whose frequency is, 220 Hz and speed is 440 m/s in, a given medium., 4. A person is listening to a tone of, 500 Hz sitting at a distance of, 450 m from the source of the, sound. What is the time interval, between successive compressions, from the source?, , The amount of sound energy passing each, second through unit area is called the, intensity of sound. We sometimes use the, ter ms, “loudness”, and, “intensity”, interchangeably, but they are not the same., Loudness is a measure of the response of the, ear to the sound. Even when two sounds are, of equal intensity, we may hear one as louder, than the other simply because our ear detects, it better., , Q, , uestion, 1. Distinguish between loudness, and intensity of sound., , 12.2.4 SPEED, , OF SOUND IN DIFFERENT, , MEDIA, , Sound propagates through a medium at a, finite speed. The sound of a thunder is heard, a little later than the flash of light is seen., , 166, , SCIENCE, , 2019-2020

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So, we can make out that sound travels with, a speed which is much less than the speed of, light. The speed of sound depends on the, properties of the medium through which it, travels. You will learn about this dependence, in higher classes. The speed of sound in a, medium depends on temperature of the, medium. The speed of sound decreases when, we go from solid to gaseous state. In any, medium as we increase the temperature, the, speed of sound increases. For example, the, speed of sound in air is 331 m s–1 at 0 ºC and, 344 m s–1 at 22 ºC. The speeds of sound at a, particular temperature in various media are, listed in Table 12.1. You need not memorise, the values., , Sonic boom: When the speed of any object, exceeds the speed of sound it is said to be, travelling at supersonic speed. Bullets, jet, aircrafts etc. often travel at supersonic, speeds. When a sound, producing source, moves with a speed higher than that of, sound, it produces shock waves in air., These shock waves carry a large amount, of energy. The air pressure variation, associated with this type of shock waves, produces a very sharp and loud sound, called the “sonic boom”. The shock waves, produced by a supersonic aircraft have, enough energy to shatter window glass and, even damage buildings., , 12.3 Reflection of Sound, Table 12.1: Speed of sound in, different media at 25 ºC, State, , Substance, , Speed in m/s, , Solids, , Aluminium, Nickel, Steel, Iron, Brass, Glass (Flint), , 6420, 6040, 5960, 5950, 4700, 3980, , Liquids Water (Sea), Water (distilled), Ethanol, Methanol, , 1531, 1498, 1207, 1103, , Gases, , 1284, 965, 346, 316, , Hydrogen, Helium, Air, Oxygen, Sulphur dioxide, , Sound bounces off a solid or a liquid like a, rubber ball bounces off a wall. Like light, sound, gets reflected at the surface of a solid or liquid, and follows the same laws of reflection as you, have studied in earlier classes. The directions, in which the sound is incident and is reflected, make equal angles with the normal to the, reflecting surface at the point of incidence, and, the three are in the same plane. An obstacle of, large size which may be polished or rough is, needed for the reflection of sound waves., , Activity _____________ 12.5, •, , •, •, , •, , 213, •, , Q, , uestion, 1. In which of the three media, air,, water or iron, does sound travel, the fastest at a particular, temperature?, , •, , Take two identical pipes, as shown in, Fig. 12.11. You can make the pipes, using chart paper. The length of the, pipes should be sufficiently long, as shown., Arrange them on a table near a wall., Keep a clock near the open end of one, of the pipes and try to hear the sound, of the clock through the other pipe., Adjust the position of the pipes so, that you can best hear the sound of, the clock., Now, measure the angles of incidence, and reflection and see the, relationship between the angles., Lift the pipe on the right vertically, to a small height and observe, what happens., (In place of a clock, a mobile phone, on vibrating mode may also be used.), 167, , SOUND, , 2019-2020

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excessive reverberation is highly undesirable., To reduce reverberation, the roof and walls of, the auditorium are generally covered with, sound-absorbent materials like compressed, fibreboard, rough plaster or draperies. The, seat materials are also selected on the basis, of their sound absorbing properties., Example 12.2 A person clapped his hands, near a cliff and heard the echo after 2 s., What is the distance of the cliff from the, person if the speed of the sound, v is, taken as 346 m s–1?, , Fig. 12.11: Reflection of sound, , 12.3.1 ECHO, If we shout or clap near a suitable reflecting, object such as a tall building or a mountain,, we will hear the same sound again a little, later. This sound which we hear is called an, echo. The sensation of sound persists in our, brain for about 0.1 s. To hear a distinct echo, the time interval between the original sound, and the reflected one must be at least 0.1s., If we take the speed of sound to be 344 m/s, at a given temperature, say at 22 ºC in air,, the sound must go to the obstacle and reach, back the ear of the listener on reflection after, 0.1s. Hence, the total distance covered by the, sound from the point of generation to the, reflecting surface and back should be at least, (344 m/s) × 0.1 s = 34.4 m. Thus, for hearing, distinct echoes, the minimum distance of the, obstacle from the source of sound must be, half of this distance, that is, 17.2 m. This, distance will change with the temperature of, air. Echoes may be heard more than once, due to successive or multiple reflections. The, rolling of thunder is due to the successive, reflections of the sound from a number of, reflecting surfaces, such as the clouds and, the land., , Solution:, Given,, Speed of sound, v = 346 m s–1, Time taken for hearing the echo,, t=2s, Distance travelled by the sound, = v × t = 346 m s–1 × 2 s = 692 m, In 2 s sound has to travel twice the, distance between the cliff and the, person. Hence, the distance between, the cliff and the person, = 692 m/2 = 346 m., , Q, , uestion, 1. An echo is heard in 3 s. What is, the distance of the reflecting, surface from the source, given, that the speed of sound is, 342 m s–1?, , 12.3.3 USES, , OF MULTIPLE REFLECTION, , OF SOUND, , 12.3.2 REVERBERATION, A sound created in a big hall will persist by, repeated reflection from the walls until it is, reduced to a value where it is no longer, audible. The repeated reflection that results, in this persistence of sound is called, reverberation. In an auditorium or big hall, , 1. Megaphones or loudhailers, horns,, musical instruments such as trumpets, and shehanais, are all designed to, send sound in a particular direction, without spreading it in all directions,, as shown in Fig 12.12., , 168, , SCIENCE, , 2019-2020

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soundboard may be placed behind the, stage so that the sound, after reflecting, from the sound board, spreads evenly, across the width of the hall (Fig 12.15)., , Megaphone, , Horn, Fig. 12.14: Curved ceiling of a conference hall., , Fig 12.12: A megaphone and a horn., , In these instruments, a tube followed, by a conical opening reflects sound, successively to guide most of the sound, waves from the source in the forward, direction towards the audience., 2. Stethoscope is a medical instrument, used for listening to sounds produced, within the body, mainly in the heart or, lungs. In stethoscopes the sound of the, patient’s heartbeat reaches the doctor’s, ears by multiple reflection of sound, as, shown in Fig.12.13., , Fig. 12.15: Sound board used in a big hall., , uestion, , Q, , Fig.12.13: Stethoscope, , 3. Generally the ceilings of concert halls,, conference halls and cinema halls are, curved so that sound after reflection, reaches all corners of the hall, as shown, in Fig 12.14. Sometimes a curved, , 1. Why are the ceilings of concert, halls curved?, , 12.4 Range of Hearing, The audible range of sound for human beings, extends from about 20 Hz to 20000 Hz (one, Hz = one cycle/s). Children under the age of, 169, , SOUND, , 2019-2020

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five and some animals, such as dogs can hear, up to 25 kHz (1 kHz = 1000 Hz). As people, grow older their ears become less sensitive to, higher frequencies. Sounds of frequencies, below 20 Hz are called infrasonic sound or, infrasound. If we could hear infrasound we, would hear the vibrations of a pendulum just, as we hear the vibrations of the wings of a, bee. Rhinoceroses communicate using, infrasound of frequency as low as 5 Hz., Whales and elephants produce sound in the, infrasound range. It is observed that some, animals get disturbed before earthquakes., Earthquakes produce low-frequency, infrasound before the main shock waves, begin which possibly alert the animals., Frequencies higher than 20 kHz are called, ultrasonic sound or ultrasound. Ultrasound, is produced by animals such as dolphins,, bats and porpoises. Moths of certain families, have very sensitive hearing equipment. These, moths can hear the high frequency squeaks, of the bat and know when a bat is flying, nearby, and are able to escape capture. Rats, also play games by producing ultrasound., , Hearing Aid: People with hearing loss may, need a hearing aid. A hearing aid is an, electronic, battery operated device. The, hearing aid receives sound through a, microphone. The microphone converts the, sound waves to electrical signals. These, electrical signals are amplified by an, amplifier. The amplified electrical signals, are given to a speaker of the hearing aid., The speaker converts the amplified, electrical signal to sound and sends to the, ear for clear hearing., , Q, , 12.5 Applications of Ultrasound, Ultrasounds are high frequency waves., Ultrasounds are able to travel along welldefined paths even in the presence of, obstacles. Ultrasounds are used extensively, in industries and for medical purposes., •, Ultrasound is generally used to clean, parts located in hard-to-reach places,, for example, spiral tube, odd shaped, parts, electronic components etc., Objects to be cleaned are placed in a, cleaning solution and ultrasonic, waves are sent into the solution. Due, to the high frequency, the particles of, dust, grease and dirt get detached and, drop out. The objects thus get, thoroughly cleaned., •, Ultrasounds can be used to detect, cracks and flaws in metal blocks., Metallic components are generally, used in construction of big structures, like buildings, bridges, machines and, also scientific equipment. The cracks, or holes inside the metal blocks, which, are invisible from outside reduces the, strength of the structure. Ultrasonic, waves are allowed to pass through the, metal block and detectors are used to, detect the transmitted waves. If there, is even a small defect, the ultrasound, gets reflected back indicating the, presence of the flaw or defect, as shown, in Fig. 12.16., , uestions, 1. What is the audible range of the, average human ear?, 2. What is the range of frequencies, associated with, (a) Infrasound?, (b) Ultrasound?, , Fig 12.16: Ultrasound is reflected back from the, defective locations inside a metal block., , 170, , SCIENCE, , 2019-2020

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Ordinary sound of longer wavelengths, cannot be used for such purpose as it will, bend around the corners of the defective, location and enter the detector., •, Ultrasonic waves are made to reflect, from various parts of the heart and, form the image of the heart. This technique is called ‘echocardiography’., •, Ultrasound scanner is an instrument, which uses ultrasonic waves for, getting images of internal organs of, the human body. A doctor may image, the patient’s organs such as the liver,, gall bladder, uterus, kidney, etc. It, helps the doctor to detect, abnormalities, such as stones in the, gall bladder and kidney or tumours, in different organs. In this technique, the ultrasonic waves travel through, the tissues of the body and get, reflected from a region where there is, a change of tissue density. These, waves are then converted into, electrical signals that are used to, generate images of the organ. These, images are then displayed on a, monitor or printed on a film. This, technique is called ‘ultrasonography’., Ultrasonography is also used for, examination of the foetus during, pregnancy to detect congenial defects, and growth abnormalities., •, Ultrasound may be employed to break, small ‘stones’ formed in the kidneys, into fine grains. These grains later get, flushed out with urine., , Fig.12.17: Ultrasound sent by the transmitter and, received by the detector., , The transmitter produces and transmits, ultrasonic waves. These waves travel through, water and after striking the object on the, seabed, get reflected back and are sensed, by the detector. The detector converts the, ultrasonic waves into electrical signals which, are appropriately interpreted. The distance, of the object that reflected the sound wave, can be calculated by knowing the speed of, sound in water and the time interval between, transmission and reception of the, ultrasound. Let the time interval between, transmission and reception of ultrasound, signal be t and the speed of sound through, seawater be v. The total distance, 2d travelled, by the ultrasound is then, 2d = v × t., The above method is called echo-ranging., The sonar technique is used to determine the, depth of the sea and to locate underwater, hills, valleys, submarine, icebergs, sunken, ship etc., , 12.5.1SONAR, The acronym SONAR stands for SOund, Navigation And Ranging. Sonar is a device, that uses ultrasonic waves to measure the, distance, direction and speed of underwater, objects. How does the sonar work? Sonar, consists of a transmitter and a detector and is, installed in a boat or a ship, as shown in Fig., 12.17., , Example 12.3 A ship sends out ultrasound, that returns from the seabed and is, detected after 3.42 s. If the speed of, ultrasound through seawater is, 1531 m/s, what is the distance of the, seabed from the ship?, Solution:, Given,, T ime between transmission and, detection, t = 3.42 s., 171, , SOUND, , 2019-2020

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12.6 Structure of Human Ear, , Speed of ultrasound in sea water,, v = 1531 m/s, Distance travelled by the ultrasound, = 2 × depth of the sea = 2d, where d is the depth of the sea., 2d = speed of sound × time, = 1531 m/s × 3.42 s = 5236 m, d = 5236 m/2 = 2618 m., Thus, the distance of the seabed from, the ship is 2618 m or 2.62 km., , Q, , How do we hear? We are able to hear with, the help of an extremely sensitive device, called the ear. It allows us to convert pressure, variations in air with audible frequencies into, electric signals that travel to the brain via the, auditory nerve. The auditory aspect of human, ear is discussed below., , uestion, 1. A submarine emits a sonar pulse,, which, returns, from, an, underwater cliff in 1.02 s. If the, speed of sound in salt water is, 1531 m/s, how far away is the, cliff?, , As mentioned earlier, bats search out prey, and fly in dark night by emitting and, detecting reflections of ultrasonic waves. The, high-pitched ultrasonic squeaks of the bat, are reflected from the obstacles or prey and, returned to bat’s ear, as shown in Fig. 12.18., The nature of reflections tells the bat where, the obstacle or prey is and what it is like., Porpoises also use ultrasound for navigation, and location of food in the dark., , Fig. 12.18: Ultrasound is emitted by a bat and it is, reflected back by the prey or an obstacle., , Fig. 12.19: Auditory parts of human ear., , The outer ear is called ‘pinna’. It collects, the sound from the surroundings. The, collected sound passes through the auditory, canal. At the end of the auditory canal there, is a thin membrane called the ear drum or, tympanic membrane. When a compression, of the medium reaches the eardrum the, pressure on the outside of the membrane, increases and forces the eardrum inward., Similarly, the eardrum moves outward when, a rarefaction reaches it. In this way the, eardrum vibrates. The vibrations are, amplified several times by three bones (the, hammer, anvil and stirrup) in the middle ear., The middle ear transmits the amplified, pressure variations received from the sound, wave to the inner ear. In the inner ear, the, pressure variations are turned into electrical, signals by the cochlea. These electrical signals, are sent to the brain via the auditory nerve,, and the brain interprets them as sound., , 172, , SCIENCE, , 2019-2020

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What, you have, learnt, •, •, •, •, •, •, , •, •, •, , Sound is produced due to vibration of different objects., Sound travels as a longitudinal wave through a material, medium., Sound travels as successive compressions and rarefactions in, the medium., In sound propagation, it is the energy of the sound that travels, and not the particles of the medium., Sound cannot travel in vacuum., The change in density from one maximum value to the, minimum value and again to the maximum value makes one, complete oscillation., The distance between two consecutive compressions or two, consecutive rarefactions is called the wavelength, λ., The time taken by the wave for one complete oscillation of the, density or pressure of the medium is called the time period, T., The number of complete oscillations per unit time is called the, frequency (ν), ν =, , •, •, •, , •, •, •, •, •, •, , 1, ., T, , The speed v, frequency ν, and wavelength λ, of sound are related, by the equation, v = λν., The speed of sound depends primarily on the nature and the, temperature of the transmitting medium., The law of reflection of sound states that the directions in which, the sound is incident and reflected make equal angles with, the normal to the reflecting surface at the point of incidence, and the three lie in the same plane., For hearing a distinct sound, the time interval between the, original sound and the reflected one must be at least 0.1 s., The persistence of sound in an auditorium is the result of, repeated reflections of sound and is called reverberation., Sound properties such as pitch, loudness and quality are, determined by the corresponding wave properties., Loudness is a physiological response of the ear to the, intensity of sound., The amount of sound energy passing each second through, unit area is called the intensity of sound., The audible range of hearing for average human beings is, in the frequency range of 20 Hz – 20 kHz., 173, , SOUND, , 2019-2020

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•, , •, •, , Sound waves with frequencies below the audible range are, termed “infrasonic” and those above the audible range are, termed “ultrasonic”., Ultrasound has many medical and industrial applications., The SONAR technique is used to determine the depth of the, sea and to locate under water hills, valleys, submarines,, icebergs, sunken ships etc., , Exercises, 1. What is sound and how is it produced?, 2. Describe with the help of a diagram, how compressions and, rarefactions are produced in air near a source of sound., 3. Cite an experiment to show that sound needs a material, medium for its propagation., 4. Why is sound wave called a longitudinal wave?, 5. Which characteristic of the sound helps you to identify your, friend by his voice while sitting with others in a dark room?, 6. Flash and thunder are produced simultaneously. But thunder, is heard a few seconds after the flash is seen, why?, 7. A person has a hearing range from 20 Hz to 20 kHz. What are, the typical wavelengths of sound waves in air corresponding, to these two frequencies? Take the speed of sound in air as, 344 m s–1., 8. Two children are at opposite ends of an aluminium rod. One, strikes the end of the rod with a stone. Find the ratio of times, taken by the sound wave in air and in aluminium to reach the, second child., 9. The frequency of a source of sound is 100 Hz. How many times, does it vibrate in a minute?, 10. Does sound follow the same laws of reflection as light does?, Explain., 11. When a sound is reflected from a distant object, an echo is, produced. Let the distance between the reflecting surface and, the source of sound production remains the same. Do you, hear echo sound on a hotter day?, 12. Give two practical applications of reflection of sound waves., 13. A stone is dropped from the top of a tower 500 m high into a, pond of water at the base of the tower. When is the splash, heard at the top? Given, g = 10 m s–2 and speed of sound =, 340 m s–1., 14. A sound wave travels at a speed of 339 m s –1 . If its, wavelength is 1.5 cm, what is the frequency of the wave?, Will it be audible?, 174, , SCIENCE, , 2019-2020

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15., 16., 17., 18., 19., 20., , What is reverberation? How can it be reduced?, What is loudness of sound? What factors does it depend on?, Explain how bats use ultrasound to catch a prey., How is ultrasound used for cleaning?, Explain the working and application of a sonar., A sonar device on a submarine sends out a signal and receives, an echo 5 s later. Calculate the speed of sound in water if the, distance of the object from the submarine is 3625 m., 21. Explain how defects in a metal block can be detected using, ultrasound., 22. Explain how the human ear works., , 175, , SOUND, , 2019-2020

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Chapter 13, WHY DO WE FALL ILL?, Activity _____________ 13.1, •, , •, , •, , •, , We have all heard of the earthquakes in, Latur, Bhuj, Kashmir etc. or the cyclones, that lashed the coastal regions. Think, of as many different ways as possible in, which people’s health would be affected, by such a disaster if it took place in our, neighbourhood., How many of these ways we can think of, are events that would occur when the, disaster is actually happening?, How many of these health-related events, would happen long after the actual, disaster, but would still be because of the, disaster?, Why would one effect on health fall into, the first group, and why would another, fall into the second group?, , When we do this exercise, we realise that, health and disease in human communities, are very complex issues, with many, interconnected causes. We also realise that, the ideas of what ‘health’ and ‘disease’ mean, are themselves very complicated. When we, ask what causes diseases and how we prevent, them, we have to begin by asking what these, notions mean., We have seen that cells are the basic units, of organisms. Cells are made of a variety of, chemical substances–proteins, carbo-hydrates,, fats or lipids, and so on. Cell is a dynamic place., Something or the other is always happening, inside them. Complex reactions and repair goes, on inside cells. New cells are being made. In, our organs or tissues, there are various, specialised activities going on– the heart is, beating, the lungs are breathing, the kidney, is filtering urine, the brain is thinking., All these activities are interconnected. For, example, if the kidneys are not filtering urine,, , poisonous substances will accumulate. Under, such conditions, the brain will not be able to, think properly. For all these interconnected, activities, energy and raw material are needed., Food is a necessity for cell and tissue functions., Anything that prevents proper functioning of, cells and tissues will lead to a lack of proper, activity of the body., It is in this context that we will look at the, notions of health and disease., , 13.1 Health and its Failure, 13.1.1 THE, , SIGNIFICANCE OF ‘HEALTH’, , We have heard the word ‘health’ used quite, frequently. We use it ourselves as well, when, we say things like ‘my grandmother’s health, is not good’. Our teachers use it when they, scold us saying ‘this is not a healthy attitude’., What does the word ‘health’ mean?, If we think about it, we realise that it, always implies the idea of ‘being well’. We can, think of this well-being as ef fective, functioning. For our grandmothers, being able, to go out to the market or to visit neighbours, is ‘being well’, and not being able to do such, things is ‘poor health’. Being interested in, following the teaching in the classroom so that, we can understand the world is called a, ‘healthy attitude’; while not being interested, is called the opposite. ‘Health’ is therefore a, state of being well enough to function well, physically, mentally and socially., , 13.1.2 PERSONAL AND COMMUNITY ISSUES, BOTH MATTER FOR HEALTH, , If health means a state of physical, mental and, social well-being, it cannot be something that, , 2019-2020

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each one of us can achieve entirely on our own., The health of all organisms will depend on, their surroundings or their environment. The, environment includes the physical, environment. So, for example, health is at risk, in a cyclone in many ways., Human beings live in societies. Our social, environment, therefore, is an important factor, in our individual health. We live in villages,, towns or cities. In such places, even our, physical environment is decided by our social, environment., Consider what would happen if no agency, is ensuring that garbage is collected and, disposed. What would happen if no one takes, responsibility for clearing the drains and, ensuring that water does not collect in the, streets or open spaces?, So, if there is a great deal of garbage, thrown in our streets, or if there is open drainwater lying stagnant around where we live,, the possibility of poor health increases., Therefore, public cleanliness is important for, individual health., , Activity _____________ 13.2, •, , •, , Find out what provisions are made by, your local authority (panchayat/, municipal corporation) for the supply, of clean drinking water., Are all the people in your locality able, to access this?, , Activity _____________ 13.3, •, , •, •, •, , Find out how your local authority, manages the solid waste generated in, your neighbourhood., Are these measures adequate?, If not, what improvements would you, suggest?, What could your family do to reduce, the amount of solid waste generated, during a day/week?, , We need food to be healthy, and this food, will have to be earned by doing work. For this,, the opportunity to do work has to be available., We need to be happy in order to be truly, healthy, and if we mistreat each other and are, afraid of each other, we cannot be happy or, , healthy. Social equality and harmony are, therefore necessary for individual health. We, can think of many other such examples of, connections between community issues and, individual health., The Five ‘F’s — What is to be done?, Protect the water source (H), Treat and store water, safely (S), Wash hands before preparing and, taking food (H), Wash hands after defecation (S), , Cover the food (H), Control flies (S), Faecal matter, , Healthy person, , Clean vegetables and fruits before, use (H), Avoid open defecation (S), , Proper drainage system (H), Treatment of water (S), , Hygiene, Sanitation, , Prevention of Transmission of Diseases by Maintaining, Sanitation and Hygiene, , 13.1.3 DISTINCTIONS BETWEEN ‘HEALTHY’, AND ‘DISEASE-FREE’, If this is what we mean by ‘health’, what do we, mean by ‘disease’? The word is actually selfexplanatory – we can think of it as ‘disease’ –, disturbed ease. Disease, in other words,, literally means being uncomfortable. However,, the word is used in a more limited meaning., We talk of disease when we can find a specific, and particular cause for discomfort. This does, not mean that we have to know the absolute, final cause; we can say that someone is, suffering from diarrhoea without knowing, exactly what has caused the loose motions., We can now easily see that it is possible to, be in poor health without actually suffering, from a particular disease. Simply not being, diseased is not the same as being healthy., ‘Good health’ for a dancer may mean being, able to stretch his body into difficult but, graceful positions. On the other hand, good, health for a musician may mean having enough, breathing capacity in his/her lungs to control, the notes from his/her flute. To have the, opportunity to realise the unique potential in, all of us is also necessary for real health., 177, , WHY DO WE FALL ILL?, , 2019-2020

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So, we can be in poor health without there, being a simple cause in the form of an, identifiable disease. This is the reason why,, when we think about health, we think about, societies and communities. On the other, hand, when we think about disease, we think, about individual sufferers., , Q, , uestions, 1. State any two conditions essential, for good health., 2. State any two conditions essential, for being free of disease., 3. Are the answers to the above, questions necessarily the same or, different? Why?, , 13.2 Disease and Its Causes, 13.2.1 WHAT DOES DISEASE LOOK LIKE?, Let us now think a little more about, diseases. In the first place, how do we know, that there is a disease? In other words, how, do we know that there is something wrong, with the body? There are many tissues in, the body, as we have seen in Chapter 6., These tissues make up physiological, systems or organ systems that carry out, body functions. Each of the organ systems, has specific organs as its parts, and it has, particular functions. So, the digestive, system has the stomach and intestines, and, it helps to digest food taken in from outside, the body. The musculoskeletal system,, which is made up of bones and muscles,, holds the body parts together and helps the, body move., When there is a disease, either the, functioning of one or more systems of the body, will change for the worse. These changes give, rise to symptoms and signs of disease., Symptoms of disease are the things we feel as, being ‘wrong’. So we have a headache, we have, cough, we have loose motions, we have a, wound with pus; these are all symptoms., These indicate that there may be a disease, but, they don’t indicate what the disease is. For, example, a headache may mean just, , examination stress or, very rarely, it may, mean meningitis, or any one of a dozen, different diseases., Signs of disease are what physicians will, look for on the basis of the symptoms. Signs, will give a little more definite indication of the, presence of a particular disease. Physicians will, also get laboratory tests done to pinpoint the, disease further., , 13.2.2 ACUTE AND CHRONIC DISEASES, The manifestations of disease will be different, depending on a number of factors. Some, diseases last for only very short periods of time,, and these are called acute diseases. We all know, from experience that the common cold lasts, only a few days. Other ailments can last for a, long time, even as much as a lifetime, and are, called chronic diseases. An example is the, infection causing elephantiasis, which is very, common in some parts of India., , Activity _____________ 13.4, •, , •, •, •, , Survey your neighbourhood to find out:, (1) how many people suffered from, acute diseases during the last, three months,, (2) how many people developed, chronic diseases during this same, period,, (3) and finally, the total number of, people suffering from chronic, diseases in your neighbourhood., Are the answers to questions (1) and, (2) different?, Are the answers to questions (2) and, (3) different?, What do you think could be the, reason for these differences? What do, you think would be the effect of these, differences on the general health of, the population?, , 13.2.3 C HRONIC, , DISEASES AND POOR, , HEALTH, , Acute and chronic diseases have different, effects on our health. Any disease that causes, poor functioning of some part of the body will, affect our health. This is because all functions, of the body are necessary for being healthy., , 178, , SCIENCE, , 2019-2020

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But an acute disease, which is over very soon,, will not have time to cause major effects on, general health, while a chronic disease will do, so., As an example, think about a cough and, cold, which all of us have from time to time., Most of us get better and become well within a, week or so. And there are no lasting effects on, our health. But if we get infected with a chronic, disease such as tuberculosis of the lungs, then, being ill over the years does make us lose, weight and feel tired all the time., We may not go to school for a few days if, we have an acute disease. But a chronic, disease will make it difficult for us to follow, what is being taught in school and reduce our, ability to learn. In other words, we are likely, to have prolonged general poor health if we, have a chronic disease. Chronic diseases, therefore, have very drastic long-term effects, on people’s health as compared to acute, diseases., , 13.2.4 CAUSES OF DISEASES, What causes disease? When we think about, causes of diseases, we must remember that, there are many levels of such causes. Let us, look at an example. If there is a baby suffering, from loose motions, we can say that the cause, of the loose motions is probably an infection., But the next question is – where did the, infection come from? Suppose we find that the, infection came through unclean drinking, water. But many babies must have had this, unclean drinking water. So, why is it that one, baby developed loose motions when the other, babies did not?, One reason might be that this baby is not, healthy. As a result, it might be more likely to, have disease when exposed to risk, whereas, healthier babies would not. Why is the baby, not healthy? Perhaps because it is not well, nourished and does not get enough food. So,, lack of good nourishment becomes a second, cause of the disease. Further, why is the baby, not well nourished? Perhaps because it is from, a household which is poor., It is also possible that the baby has some, genetic difference that makes it more likely to, suffer from loose motions when exposed to a, pathogen. Without the pathogen, the genetic, , difference or the poor nourishment alone, would not lead to loose motions. But they do, become contributory causes of the disease., Why was there no clean drinking water for, the baby? Perhaps because the public services, are poor where the baby’s family lives. So,, poverty or lack of public services become third, cause of the baby’s disease., It will now be obvious that all diseases will, have immediate causes and contributory, causes. Also, most diseases will have many, causes, rather than one single cause., , 13.2.5 INFECTIOUS AND NON-INFECTIOUS, CAUSES, , As we have seen, it is important to keep public, health and community health factors in mind, when we think about causes of diseases. We, can take that approach a little further. It is, useful to think of the immediate causes of, disease as belonging to two distinct types. One, group of causes is the infectious agents,, mostly micr obes or micro-organisms., Diseases where microbes are the immediate, causes are called infectious diseases. This is, because the microbes can spread in the, community, and the diseases they cause will, spread with them., Things to ponder, 1. Do all diseases spread to people, coming in contact with a sick person?, 2. What are the diseases that are not, spreading?, 3. How would a person develop those, diseases that don’t spread by contact, with a sick person?, On the other hand, there are also diseases, that are not caused by infectious agents. Their, causes vary, but they are not external causes, like microbes that can spread in the, community. Instead, these are mostly internal,, non-infectious causes., For example, some cancers are caused by, genetic abnormalities. High blood pressure, can be caused by excessive weight and lack, of exercise. You can think of many other, diseases where the immediate causes will not, be infectious., 179, , WHY DO WE FALL ILL?, , 2019-2020

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Peptic ulcers and the Nobel prize, For many years, everybody used to think, that peptic ulcers, which cause acidity–, related pain and bleeding in the stomach, and duodenum, were because of lifestyle, reasons. Everybody thought that a stressful, life led to a lot of acid secretion in the, stomach, and eventually caused peptic, ulcers., Then two Australians made a discovery, that a bacterium, Helicobacter pylori, was, responsible for peptic ulcers. Robin Warren, (born 1937), a pathologist from Perth,, Australia, saw these small curved bacteria, in the lower part of the stomach in many, patients. He noticed that signs of, inflammation were always present around, these bacteria. Barry Marshall (born 1951),, a young clinical fellow, became interested, in Warren’s findings and succeeded in, cultivating the bacteria from these sources., In treatment studies, Marshall and, Warren showed that patients could be cured, of peptic ulcer only when the bacteria were, killed off from the stomach. Thanks to this, pioneering discovery by Marshall and, Warren, peptic ulcer disease is no longer a, chronic, frequently disabling condition, but, a disease that can be cured by a short, period of treatment with antibiotics., For this achievement, Marshall and, , Warren (seen in the picture) received the, Nobel prize for physiology and medicine in, 2005., , The ways in which diseases spread, and, the ways in which they can be treated and, prevented at the community level would be, different for different diseases. This would, depend a lot on whether the immediate causes, are infectious or non-infectious., , Q, , uestions, 1. List any three reasons why you, would think that you are sick and, ought to see a doctor. If only one, of these symptoms were present,, would you still go to the doctor?, Why or why not?, 2. In which of the following case do, you think the long-term effects on, your health are likely to be most, unpleasant?, • if you get jaundice,, • if you get lice,, • if you get acne., Why?, , 13.3 Infectious Diseases, 13.3.1 INFECTIOUS AGENTS, We have seen that the entire diversity seen in, the living world can be classified into a few, groups. This classification is based on, common characteristics between different, organisms. Organisms that can cause disease, are found in a wide range of such categories, of classification. Some of them are viruses,, some are bacteria, some are fungi, some are, single-celled animals or protozoans (Fig. 13.1)., Some diseases are also caused by, multicellular organisms, such as worms of, different kinds., , 180, , SCIENCE, , 2019-2020

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Fig. 13.1(a): Picture of SARS viruses coming out (see, arrows for examples) of the surface of, an infected cell. The white scale line, represents 500 nanometres, which is, half a micrometre, which is onethousandth of a millimetre. The scale line, gives us an idea of how small the things, we are looking at are., Courtesy:, Emerging Infectious, Deseases, a journal of CDC, U.S., , Fig. 13.1(d): Picture of Leishmania, the protozoan, organism that causes kala-azar. The, organisms are oval-shaped, and each, has one long whip-like structure. One, organism (arrow) is dividing, while a cell, of the immune system (lower right) has, gripped on the two whips of the dividing, organism and is sending cell processes, up to eat up the organism. The immune, cell is about ten micrometres in diameter., , Fig. 13.1(b): Picture of staphylococci, the bacteria, which can cause acne. The scale of the, image is indicated by the line at top left,, which is 5 micrometres long., , Fig. 13.1(c): Picture of Trypanosoma, the protozoan, organism responsible for sleeping, sickness. The organism is lying next to, a saucer-shaped red blood cell to give, an idea of the scale., Copyright: Oregon Health and Science, University, U.S., , Fig. 13.1(e): Picture of an adult roundworm (Ascaris, lumbricoides) from the small intestine. The, ruler next to it shows four centimetres to, give us an idea of the scale., , 181, , WHY DO WE FALL ILL?, , 2019-2020

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Common examples of diseases caused by, viruses are the common cold, influenza,, dengue fever and AIDS. Diseases like typhoid, fever, cholera, tuberculosis and anthrax are, caused by bacteria. Many common skin, infections are caused by different kinds of, fungi. Protozoan microbes cause many, familiar diseases, such as malaria and kalaazar. All of us have also come across intestinal, worm infections, as well as diseases like, elephantiasis caused by diffferent species of, worms., Why is it important that we think of these, categories of infectious agents? The answer, is that these categories are important factors, in deciding what kind of treatment to use., Members of each one of these groups –, viruses, bacteria, and so on – have many, biological characteristics in common., All viruses, for example, live inside host, cells, whereas bacteria very rarely do. Viruses,, bacteria and fungi multiply very quickly, while, worms multiply very slowly in comparison., Taxonomically, all bacteria are closely related, to each other than to viruses and vice versa., This means that many important life, processes are similar in the bacteria group, but are not shared with the virus group. As a, result, drugs that block one of these life, processes in one member of the group is likely, to be effective against many other members, of the group. But the same drug will not work, against a microbe belonging to a different, group., As an example, let us take antibiotics. They, commonly block biochemical pathways, important for bacteria. Many bacteria, for, example, make a cell-wall to protect, themselves. The antibiotic penicillin blocks, the bacterial processes that build the cellwall. As a result, the growing bacteria become, unable to make cell-walls, and die easily., Human cells don’t make a cell-wall anyway,, so penicillin cannot have such an effect on us., Penicillin will have this effect on any bacteria, that use such processes for making cell-walls., Similarly, many antibiotics work against many, species of bacteria rather than simply working, against one., , But viruses do not use these pathways at, all, and that is the reason why antibiotics do, not work against viral infections. If we have a, common cold, taking antibiotics does not, reduce the severity or the duration of the, disease. However, if we also get a bacterial, infection along with the viral cold, taking, antibiotics will help. Even then, the antibiotic, will work only against the bacterial part of, the infection, not the viral infection., , Activity _____________ 13.5, •, •, •, •, •, •, •, , Find out how many of you in your, class had cold/cough/fever recently., How long did the illness last?, How many of you took antibiotics (ask, your parents if you had antibiotics)?, How long were those who took, antibiotics ill?, How long were those who didn’t take, antibiotics ill?, Is there a difference between these two, groups?, If yes, why? If not, why not?, , 13.3.2 MEANS OF SPREAD, How do infectious diseases spread? Many, microbial agents can commonly move from, an affected person to someone else in a variety, of ways. In other words, they can be, ‘communicated’, and so are also called, communicable diseases., Such disease-causing microbes can spread, through the air. This occurs through the little, droplets thrown out by an infected person who, sneezes or coughs. Someone standing close by, can breathe in these droplets, and the microbes, get a chance to start a new infection. Examples, of such diseases spread through the air are, the common cold, pneumonia and, tuberculosis., We all have had the experience of sitting, near someone suffering from a cold and, catching it ourselves. Obviously, the more, crowded our living conditions are, the more, likely it is that such airborne diseases will, spread., , 182, , SCIENCE, , 2019-2020

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Diseases can also be spread through, water. This occurs if the excreta from someone, suffering from an infectious gut disease, such, as cholera, get mixed with the drinking water, used by people living nearby. The choleracausing microbes will enter a healthy person, through the water they drink and cause, disease in them. Such diseases are much more, likely to spread in the absence of safe supplies, of drinking water., The sexual act is one of the closest physical, contact two people can have with each other., Not surprisingly, there are microbial infections, such as syphilis or AIDS that are transmitted, by sexual contact from one partner to the, other. However, such sexually transmitted, diseases are not spread by casual physical, contact. Casual physical contacts include, handshakes or hugs or sports, like wrestling,, or by any of the other ways in which we touch, each other socially. Other than the sexual, contact, the virus causing AIDS (HIV) can also, spread through blood-to-blood contact with, infected people or from an infected mother to, her baby during pregnancy or through breast, feeding., We live in an environment that is full of, many other creatures apart from us. It is, inevitable that many diseases will be, transmitted by other animals. These animals, carry the infecting agents from a sick person, to another potential host. These animals are, thus the intermediaries and are called vectors., , Fig. 13.2: Common methods of transmission of, diseases., , The commonest vectors we all know are, mosquitoes. In many species of mosquitoes,, the females need highly nutritious food in the, form of blood in order to be able to lay mature, eggs. Mosquitoes feed on many warm-blooded, animals, including us. In this way, they can, transfer diseases from person to person, (Fig. 13.2)., , 13.3.3 O RGAN - SPECIFIC, , AND TISSUE SPECIFIC MANIFESTATIONS, , The disease-causing microbes enter the body, through these different means. Where do they, go then? The body is very large when, compared to the microbes. So there are many, possible places, organs or tissues, where they, could go. Do all microbes go to the same tissue, or organ, or do they go to different ones?, Different species of microbes seem to have, evolved to home in on different parts of the, body. In part, this selection is connected to, their point of entry. If they enter from the air, via the nose, they are likely to go to the lungs., This is seen in the bacteria causing, tuberculosis. If they enter through the mouth,, they can stay in the gut lining like typhoidcausing bacteria. Or they can go to the liver,, like the viruses that cause jaundice., But this needn’t always be the case. An, infection like HIV, that comes into the body, via the sexual organs, will spread to lymph, nodes all over the body. Malaria-causing, microbes, entering through a mosquito bite,, will go to the liver, and then to the red blood, cells. The virus causing Japanese encephalitis,, or brain fever, will similarly enter through a, mosquito bite. But it goes on to infect the brain., The signs and symptoms of a disease will, thus depend on the tissue or organ which the, microbe targets. If the lungs are the targets,, then symptoms will be cough and, breathlessness. If the liver is targeted, there will, be jaundice. If the brain is the target, we will, observe headaches, vomiting, fits or, unconsciousness. We can imagine what the, symptoms and signs of an infection will be if, we know what the target tissue or organ is,, and the functions that are carried out by this, tissue or organ., 183, , WHY DO WE FALL ILL?, , 2019-2020

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In addition to these tissue-specific effects, of infectious disease, there will be other common, effects too. Most of these common effects depend, on the fact that the body’s immune system is, activated in response to infection. An active, immune system recruits many cells to the, affected tissue to kill off the disease-causing, microbes. This recruitment process is called, inflammation. As a part of this process, there, are local effects such as swelling and pain, and, general effects such as fever., In some cases, the tissue-specificity of the, infection leads to very general-seeming effects., For example, in HIV infection, the virus goes, to the immune system and damages its, function. Thus, many of the effects of HIV-AIDS, are because the body can no longer fight off, the many minor infections that we face, everyday. Instead, every small cold can become, pneumonia. Similarly, a minor gut infection, can produce major diarrhoea with blood loss., Ultimately, it is these other infections that kill, people suffering from HIV-AIDS., It is also important to remember that the, severity of disease manifestations depend on, the number of microbes in the body. If the, number of microbes is very small, the disease, manifestations may be minor or unnoticed., But if the number is of the same microbe large,, the disease can be severe enough to be lifethreatening. The immune system is a major, factor that determines the number of microbes, surviving in the body. We shall look into this, aspect a little later in the chapter., , 13.3.4 PRINCIPLES OF TREATMENT, What are the steps taken by your family when, you fall sick? Have you ever thought why you, sometimes feel better if you sleep for some time?, When does the treatment involve medicines?, Based on what we have learnt so far, it, would appear that there are two ways to treat, an infectious disease. One would be to reduce, the effects of the disease and the other to kill, the cause of the disease. For the first, we can, provide treatment that will reduce the, symptoms. The symptoms are usually, because of inflammation. For example, we can, take medicines that bring down fever, reduce, pain or loose motions. We can take bed rest so, , that we can conserve our energy. This will, enable us to have more of it available to focus, on healing., But this kind of symptom-directed, treatment by itself will not make the infecting, microbe go away and the disease will not be, cured. For that, we need to be able to kill off, the microbes., How do we kill microbes? One way is to, use medicines that kill microbes. We have seen, earlier that microbes can be classified into, different categories. They are viruses, bacteria,, fungi or protozoa. Each of these groups of, organisms will have some essential, biochemical life process which is peculiar to, that group and not shared with the other, groups. These processes may be pathways for, the synthesis of new substances or respiration., These pathways will not be used by us, either. For example, our cells may make new, substances by a mechanism different from that, used by bacteria. We have to find a drug that, blocks the bacterial synthesis pathway without, affecting our own. This is what is achieved by, the antibiotics that we are all familiar with., Similarly, there are drugs that kill protozoa, such as the malarial parasite., One reason why making anti-viral, medicines is harder than making antibacterial medicines is that viruses have few, biochemical mechanisms of their own. They, enter our cells and use our machinery for their, life processes. This means that there are, relatively few virus-specific targets to aim at., Despite this limitation, there are now effective, anti-viral drugs, for example, the drugs that, keep HIV infection under control., , 13.3.5 PRINCIPLES OF PREVENTION, All of what we have talked about so far deals, with how to get rid of an infection in someone, who has the disease. But there are three, limitations of this approach to dealing with, infectious disease. The first is that once, someone has a disease, their body functions, are damaged and may never recover, completely. The second is that treatment will, take time, which means that someone, suffering from a disease is likely to be, , 184, , SCIENCE, , 2019-2020

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bedridden for some time even if we can give, proper treatment. The third is that the person, suffering from an infectious disease can serve, as the source from where the infection may, spread to other people. This leads to the, multiplication of the above difficulties. It is, because of such reasons that prevention of, diseases is better than their cure., How can we prevent diseases? There are, two ways, one general and one specific to each, disease. The general ways of preventing, infections mostly relate to preventing, exposure. How can we prevent exposure to, infectious microbes?, If we look at the means of their spreading,, we can get some easy answers. For airborne, microbes, we can prevent exposure by, providing living conditions that are not, overcrowded. For water-borne microbes, we, can prevent exposure by providing safe, drinking water. This can be done by treating, the water to kill any microbial contamination., For vector-borne infections, we can provide, clean environments. This would not, for, example, allow mosquito breeding. In other, words, public hygiene is one basic key to the, prevention of infectious diseases., In addition to these issues that relate to, the environment, there are some other general, principles to prevent infectious diseases. To, appreciate those principles, let us ask a, question we have not looked at so far. Normally,, we are faced with infections everyday. If, someone is suffering from a cold and cough in, the class, it is likely that the children sitting, around will be exposed to the infection. But, all of them do not actually suffer from the, disease. Why not?, This is because the immune system of our, body is normally fighting off microbes. We have, cells that specialise in killing infecting, microbes. These cells go into action each time, infecting microbes enter the body. If they are, successful, we do not actually come down with, any disease. The immune cells manage to kill, off the infection long before it assumes major, proportions. As we noted earlier, if the number, of the infecting microbes is controlled, the, manifestations of disease will be minor. In other, words, becoming exposed to or infected with, , an infectious microbe does not necessarily, mean developing noticeable disease., So, one way of looking at severe infectious, diseases is that it represents a lack of success, of the immune system. The functioning of the, immune system, like any other system in our, body, will not be good if proper and sufficient, nourishment and food is not available., Therefore, the second basic principle of, prevention of infectious disease is the, availability of proper and sufficient food for, everyone., , Activity _____________ 13.6, •, , •, •, , Conduct a survey in your locality., Talk to ten families who are well-off, and ten who are very poor (in your, estimation). Both sets of families, should have children who are below, five years of age. Measure the heights, of these children. Draw a graph of the, height of each child against its age, for both sets of families., Is there a difference between the, groups? If yes, why?, If there is no difference, do you think, that your findings mean that being, well-off or poor does not matter for, health?, , These are the general ways of preventing, infections. What are the specific ways? They, relate to a peculiar property of the immune, system that usually fights off microbial, infections. Let us cite an example to try and, understand this property., These days, there is no smallpox anywhere, in the world. But as recently as a hundred, years ago, smallpox epidemics were not at all, uncommon. In such an epidemic, people used, to be very afraid of coming near someone, suffering from the disease since they were, afraid of catching the disease., However, there was one group of people, who did not have this fear. These people would, provide nursing care for the victims of, smallpox. This was a group of people who had, had smallpox earlier and survived it, although, with a lot of scarring. In other words, if you, had smallpox once, there was no chance of, 185, , WHY DO WE FALL ILL?, , 2019-2020

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suffering from it again. So, having the disease, once was a means of preventing subsequent, attacks of the same disease., This happens because when the immune, system first sees an infectious microbe, it, responds against it and then remembers it, specifically. So the next time that particular, microbe, or its close relatives enter the body,, the immune system responds with even greater, vigour. This eliminates the infection even more, quickly than the first time around. This is the, basis of the principle of immunisation., Immunisation, Traditional Indian and Chinese medicinal, systems sometimes deliberately rubbed the, skin crusts from smallpox victims into the, skin of healthy people. They thus hoped to, induce a mild form of smallpox that would, create resistance against the disease., Famously, two centuries ago, an English, physician named Edward Jenner, realised, that milkmaids, who had had, cowpox did not, catch smallpox, even, during, e p i d e m i c s ., Cowpox is a very, mild, disease., Jenner, tried, deliberately giving, cowpox to people, (as he can be seen, doing, in, the, picture), and found that they were now, resistant to smallpox. This was because the, smallpox virus is closely related to the, cowpox virus. ‘Cow’ is ‘vacca’ in Latin, and, cowpox is ‘vaccinia’. From these roots, the, word ‘vaccination’ has come into our usage., , to the infecting microbe from turning into, actual disease., Many such vaccines are now available for, preventing a whole range of infectious diseases,, and provide a disease-specific means of, prevention. There are vaccines against tetanus,, diphtheria, whooping cough, measles, polio, and many others. These form the public health, programme of childhood immunisation for, preventing infectious diseases., Of course, such a programme can be, useful only if such health measures are, available to all children. Can you think of, reasons why this should be so?, Some hepatitis viruses, which cause, jaundice, are transmitted through water. There, is a vaccine for one of them, hepatitis A, in the, market. But the majority of children in many, parts of India are already immune to hepatitis, A by the time they are five years old. This is, because they are exposed to the virus through, water. Under these circumstances, would you, take the vaccine?, , Activity _____________ 13.7, •, , Q, , We can now see that, as a general principle,, we can ‘fool’ the immune system into, developing a memory for a particular infection, by putting something, that mimics the microbe, we want to vaccinate against, into the body., This does not actually cause the disease but, this would prevent any subsequent exposure, 186, , Rabies virus is spread by the bite of, infected dogs and other animals. There, are anti-rabies vaccines for both humans, and animals. Find out the plan of your, local authority for the control of rabies, in your neighbourhood. Are these, measures adequate? If not, what, improvements would you suggest?, , uestions, 1. Why are we normally advised to, take bland and nourishing food, when we are sick?, 2. What are the different means by, which infectious diseases are, spread?, 3. What precautions can you take in, your school to reduce the, incidence of infectious diseases?, 4. What is immunisation?, 5. What are the immunisation, programmes available at the, nearest health centre in your, locality? Which of these diseases, are the major health problems in, your area?, SCIENCE, , 2019-2020

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What, you have, learnt, •, •, •, •, •, •, •, •, •, •, •, , Health is a state of physical, mental and social well-being., The health of an individual is dependent on his/her physical, surroundings and his/her economic status., Diseases are classified as acute or chronic, depending on, their duration., Disease may be due to infectious or non-infectious causes., Infectious agents belong to different categories of organisms, and may be unicellular and microscopic or multicellular., The category to which a disease-causing organism belongs, decides the type of treatment., Infectious agents are spread through air, water, physical, contact or vectors., Prevention of disease is more desirable than its successful, treatment., Infectious diseases can be prevented by public health hygiene, measures that reduce exposure to infectious agents., Infectious diseases can also be prevented by using, immunisation., Effective prevention of infectious diseases in the community, requires that everyone should have access to public hygiene, and immunisation., , Exercises, 1. How many times did you fall ill in the last one year? What, were the illnesses?, (a) Think of one change you could make in your habits in, order to avoid any of/most of the above illnesses., (b) Think of one change you would wish for in your, surroundings in order to avoid any of/most of the above, illnesses., 2. A doctor/nurse/health-worker is exposed to more sick people, than others in the community. Find out how she/he avoids, getting sick herself/himself., 3. Conduct a survey in your neighbourhood to find out what, the three most common diseases are. Suggest three steps, that could be taken by your local authorities to bring down, the incidence of these diseases., , 187, , WHY DO WE FALL ILL?, , 2019-2020

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4. A baby is not able to tell her/his caretakers that she/he is, sick. What would help us to find out, (a) that the baby is sick?, (b) what is the sickness?, 5. Under which of the following conditions is a person most likely, to fall sick?, (a) when she is recovering from malaria., (b) when she has recovered from malaria and is taking care of, someone suffering from chicken-pox., (c) when she is on a four-day fast after recovering from, malaria and is taking care of someone suffering from, chicken-pox., Why?, 6. Under which of the following conditions are you most likely, to fall sick?, (a) when you are taking examinations., (b) when you have travelled by bus and train for two days., (c) when your friend is suffering from measles., Why?, , 188, , SCIENCE, , 2019-2020

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Chapter 14, NATURAL RESOURCES, Our planet, Earth is the only one on which, life, as we know it, exists. Life on Earth is, dependent on many factors. Most life-forms, we know need an ambient temperature,, water, and food. The resources available on, the Earth and the energy from the Sun are, necessary to meet the basic requirements of, all life-forms on the Earth., What are these resources on the Earth?, These are the land, the water and the air., The outer crust of the Earth is called the, lithosphere. Water covers 75% of the Earth’s, surface. It is also found underground. These, comprise the hydrosphere. The air that covers, the whole of the Earth like a blanket, is called, the atmosphere. Living things are found, where these three exist. This life-supporting, zone of the Earth where the atmosphere, the, hydrosphere and the lithosphere interact and, make life possible, is known as the biosphere., Living things constitute the biotic, component of the biosphere. The air, the, water and the soil form the non-living or, abiotic component of the biosphere. Let us, study these abiotic components in detail in, order to understand their role in sustaining, life on Earth., , 14.1 The Breath of Life: Air, We have already talked about the composition, of air in the first chapter. It is a mixture of, many gases like nitrogen, oxygen, carbon, dioxide and water vapour. It is interesting to, note that even the composition of air is the, result of life on Earth. In planets such as, Venus and Mars, where no life is known to, exist, the major component of the atmosphere, is found to be carbon dioxide. In fact, carbon, , dioxide constitutes up to 95-97% of the, atmosphere on Venus and Mars., Eukaryotic cells and many prokaryotic, cells, discussed in Chapter 5, need oxygen to, break down glucose molecules and get energy, for their activities. This results in the, production of carbon dioxide. Another process, which results in the consumption of oxygen, and the concomitant production of carbon, dioxide is combustion. This includes not just, human activities, which burn fuels to get, energy, but also forest fires., Despite this, the percentage of carbon, dioxide in our atmosphere is a mere fraction, of a percent because carbon dioxide is ‘fixed’, in two ways: (i) Green plants convert carbon, dioxide into glucose in the presence of, Sunlight and (ii) many marine animals use, carbonates dissolved in sea-water to make, their shells., , 14.1.1 THE ROLE OF THE ATMOSPHERE, IN CLIMATE CONTROL, , We have talked of the atmosphere covering the, Earth, like a blanket. We know that air is a, bad conductor of heat. The atmosphere keeps, the average temperature of the Earth fairly, steady during the day and even during the, course of the whole year. The atmosphere, prevents the sudden increase in temperature, during the daylight hours. And during the, night, it slows down the escape of heat into, outer space. Think of the moon, which is, about the same distance from the Sun that, the Earth is. Despite that, on the surface of, the moon, with no atmosphere, the, temperature ranges from –190° C to 110° C., , 2019-2020

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the heating of water bodies and the activities, of living organisms. The atmosphere can be, heated from below by the radiation that is, reflected back or re-radiated by the land or, water bodies. On being heated, convection, currents are set up in the air. In order to gain, some understanding of the nature of, convection currents, let us perform the, following activity:, , Activity _____________ 14.1, Measure the temperatur e of the, following :, Take (i) a beaker full of water, (ii) a, beaker full of soil/sand and (iii) a closed, bottle containing a thermometer. Keep, them in bright Sunlight for three hours., Now measure the temperature of all 3, vessels. Also, take the temperature, reading in shade at the same time., , •, , Activity _____________ 14.2, , Now answer, 1., 2., , 3., , 4., , •, , Is the temperature reading more in, activity (i) or (ii)?, Based on the above finding, which, would become hot faster – the land or, the sea?, Is the thermometer reading of the, temperature of air (in shade) the same, as the temperature of sand or water?, What do you think is the reason for, this? And why does the temperature, have to be measured in the shade?, Is the temperature of air in the closed, glass vessel/bottle the same as the, temperature taken in open air? (i) What, do you think is the reason for this?, (ii) Do we ever come across this, phenomenon in daily life?, , •, , •, , •, , Place a candle in a beaker or widemouthed bottle and light it. Light an, incense stick and take it to the mouth, of the above bottle (Figure 14.1)., Which way does the smoke flow when, the incense stick is kept near the edge, of the mouth?, Which way does the smoke flow when, the incense stick is kept a little above, the candle?, Which way does the smoke flow when, the incense stick is kept in other, regions?, , As we have seen above, sand and water, do not heat up at the same rate. What do you, think will be their rates of cooling? Can we, think of an experiment to test the prediction?, , 14.1.2 THE MOVEMENT OF AIR: WINDS, We have all felt the relief brought by cool, evening breezes after a hot day. And, sometimes, we are lucky enough to get rains, after some days of really hot weather. What, causes the movement of air, and what decides, whether this movement will be in the form of, a gentle breeze, a strong wind or a terrible, storm? What brings us the welcome rains?, All these phenomena are the result of, changes that take place in our atmosphere, due to the heating of air and the formation of, water vapour. Water vapour is formed due to, , Fig. 14.1: Air currents being caused by the uneven, heating of air., , The patterns revealed by the smoke show, us the directions in which hot and cold air, move. In a similar manner, when air is heated, by radiation from the heated land or water, it, rises. But since land gets heated faster than, water, the air over land would also be heated, faster than the air over water bodies., So, if we look at the situation in coastal, regions during the day, the air above the land, SCIENCE, , 190, , 2019-2020

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gets heated faster and starts rising. As this, air rises, a region of low pressure is created, and air over the sea moves into this area of, low pressure. The movement of air from one, region to the other creates winds. During the, day, the direction of the wind would be from, the sea to the land., At night, both land and sea start to cool., Since water cools down slower than the land,, the air above water would be warmer than, the air above land., On the basis of the above discussion, what, can you say about:, 1. the appearance of areas of low and, high pressure in coastal areas at night?, 2. the direction in which air would flow, at night in coastal areas?, Similarly, all the movements of air, resulting in diverse atmospheric phenomena, are caused by the uneven heating of the, atmosphere in different regions of the Earth., But various other factors also influence these, winds – the rotation of the Earth and the, presence of mountain ranges in the paths of, the wind are a couple of these factors. We, will not go into these factors in detail in this, chapter, but think about this: how do the, presence of the Himalayas change the flow of, a wind blowing from Allahabad towards the, north?, , 14.1.3 RAIN, Let us go back now to the question of how, clouds are formed and bring us rain. We could, start by doing a simple experiment which, demonstrates some of the factors influencing, these climatic changes., , Activity _____________ 14.3, •, , •, , Take an empty bottle of the sort in, which bottled water is sold. Pour about, 5-10 mL of water into it and close the, bottle tightly. Shake it well or leave it, out in the Sun for ten minutes. This, causes the air in the bottle to be, saturated with water vapour., Now, take a lighted incense stick. Open, the cap of the bottle and allow some of, the smoke from the incense stick to, , enter the bottle. Quickly close the bottle, once more. Make sure that the cap is, fitting tightly. Press the bottle hard, between your hands and crush it as, much as possible. Wait for a few, seconds and release the bottle. Again, press the bottle as hard as you can., , Now answer, 1., 2., 3., 4., 5., 6., , When did you observe that the air, inside seemed to become ‘foggy’?, When does this fog disappear?, When is the pressure inside the bottle, higher?, Is the ‘fog’ observed when the pressure, in the bottle is high or when it is low?, What is the need for smoke particles, inside the bottle for this experiment?, What might happen if you do the, experiment without the smoke from the, incense stick? Now try it and check if, the prediction was correct. What might, be happening in the above experiment, in the absence of smoke particles?, , The above experiment replicates, on a very, small scale, what happens when air with a, very high content of water vapour goes from, a region of high pressure to a region of low, pressure or vice versa., When water bodies are heated during the, day, a large amount of water evaporates and, goes into the air. Some amount of water, vapour also get into the atmosphere because, of various biological activities. This air also, gets heated. The hot air rises up carrying the, water vapour with it. As the air rises, it, expands and cools. This cooling causes the, water vapour in the air to condense in the, form of tiny droplets. This condensation of, water is facilitated if some particles could act, as the ‘nucleus’ for these drops to form, around. Normally dust and other suspended, particles in the air perform this function., Once the water droplets are formed, they, grow bigger by the ‘condensation’ of these, water droplets. When the drops have grown, big and heavy, they fall down in the form of, rain. Sometimes, when the temperature of air, , NATURAL RESOURCES, , 191, , 2019-2020

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is low enough, precipitation may occur in the, form of snow, sleet or hail., Rainfall patterns are decided by the, prevailing wind patterns. In large parts of, India, rains are mostly brought by the southwest or north-east monsoons. We have also, heard weather reports that say ‘depressions’, in the Bay of Bengal have caused rains in, some areas (Figure 14.2)., , Fig. 14.2: Satellite picture showing clouds over India., , Activity _____________ 14.4, •, , •, •, •, , Collect information from newspapers, or weather reports on television about, rainfall patterns across the country., Also find out how to construct a raingauge and make one. What precautions, are necessary in order to get reliable, data from this rain-gauge? Now answer, the following questions :, In which month did your city/town/, village get the maximum rainfall?, In which month did your state/union, territory get the maximum rainfall?, Is rain always accompanied by thunder, and lightning? If not, in which season, do you get more of thunder and, lightning with the rain?, , 14.1.4 AIR POLLUTION, We keep hearing of the increasing levels of, oxides of nitrogen and sulphur in the news., People often bemoan the fact that the quality, of air has gone down since their childhood., How is the quality of air affected and how, does this change in quality affect us and other, life forms?, The fossil fuels like coal and petroleum, contain small amounts of nitrogen and, sulphur. When these fuels are burnt, nitrogen, and sulphur too are burnt and this produces, different oxides of nitrogen and sulphur. Not, only is the inhalation of these gases, dangerous, they also dissolve in rain to give, rise to acid rain. The combustion of fossil fuels, also increases the amount of suspended, particles in air. These suspended particles, could be unbur nt carbon particles or, substances called hydrocarbons. Presence of, high levels of all these pollutants cause, visibility to be lowered, especially in cold, weather when water also condenses out of, air. This is known as smog and is a visible, indication of air pollution. Studies have, shown that regularly breathing air that, contains any of these substances increases, the incidence of allergies, cancer and heart, diseases. An increase in the content of these, harmful substances in air is called air, pollution., , Activity _____________ 14.5, •, , Find out more about monsoons and, cyclones from the library. Try and find, out the rainfall pattern of any other, country. Is the monsoon responsible for, rains the world over?, , Fig. 14.3: Lichen, SCIENCE, , 192, , 2019-2020

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Activity _____________ 14.6, •, , •, •, , Organisms called lichens are found to, be very sensitive to the levels of, contaminants like sulphur dioxide in, the air. As discussed earlier in section, 7.3.3, lichens can be commonly found, growing on the barks of trees as a thin, greenish-white crust. See if you can, find lichen growing on the trees in your, locality., Compare the lichen on trees near busy, roads and trees some distance away., On the trees near roads, compare the, incidence of lichen on the side facing, the road and on the side away from the, road., , What can you say about the levels of, polluting substances near roads and away, from roads on the basis of your findings, above?, , Q, , uestions, 1. How is our atmosphere different, from the atmospheres on Venus, and Mars?, 2. How does the atmosphere act as, a blanket?, 3. What causes winds?, 4. How are clouds formed?, 5. List any three human activities, that you think would lead to air, pollution., , people are forced to spend considerable, amounts of time in fetching water from faraway sources., , Activity _____________ 14.7, •, , •, , Many municipal corporations are trying, water -harvesting techniques to, improve the availability of water., Find out what these techniques are and, how they would increase the water that, is available for use., , But why is water so necessary? And do, all organisms require water? All cellular, processes take place in a water medium. All, the reactions that take place within our body, and within the cells occur between, substances that are dissolved in water., Substances are also transported from one, part of the body to the other in a dissolved, form. Hence, organisms need to maintain the, level of water within their bodies in order to, stay alive. Terrestrial life-forms require fresh, water for this because their bodies cannot, tolerate or get rid of the high amounts of, dissolved salts in saline water. Thus, water, sources need to be easily accessible for, animals and plants to survive on land., , Activity _____________ 14.8, •, , 14.2 Water: A Wonder Liquid, Water occupies a very large area of the Earth’s, surface and is also found underground. Some, amount of water exists in the form of water, vapour in the atmosphere. Most of the water, on Earth’s surface is found in seas and oceans, and is saline. Fresh water is found frozen in, the ice-caps at the two poles and on snowcovered mountains. The underground water, and the water in rivers, lakes and ponds is, also fresh. However, the availability of fresh, water varies from place to place. Practically, every summer, most places have to face a, shortage of water. And in rural areas, where, water supply systems have not been installed,, NATURAL RESOURCES, , •, , •, , Select a small area (say, 1 m2) near a, water-body, it may be a river, stream,, lake or pond. Count the number of, different animals and plants in this, area. Also, check the number of, individuals of each type or species., Compare this with the number of, individuals (both animals and plants), found in an area of the same size in a, dry, rocky region., Is the variety of plant and animal life, the same in both these areas?, , Activity _____________ 14.9, •, , •, , Select and mark out a small area (about, 1 m2) in some unused land in or near, your school., As in the above activity, count the, number of different animals and plants, in this area and the number of, individuals of each species., 193, , 2019-2020

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Remember to do this in the same place, twice in a year, once during summer, or the dry season and once after it has, rained., , •, , Now answer, 1., 2., 3., , Were the numbers similar both times?, In which season did you find more, variety of plants and animals?, In which season did you find more, number of individuals of each variety?, , After compiling the results of the above, two activities, think if there is any relationship, between the amount of available water and, the number and variety of plants and animals, that can live in a given area. If there is a, relationship, where do you think you would, find a greater variety and abundance of life –, in a region that receives 5 cm of rainfall in a, year or a region that receives 200 cm of, rainfall in a year? Find the map showing, rainfall patterns in the atlas and predict, which States in India would have the, maximum biodiversity and which would have, the least. Can we think of any way of checking, whether the prediction is correct?, The availability of water decides not only, the number of individuals of each species that, are able to survive in a particular area, but it, also decides the diversity of life there. Of, course, the availability of water is not the only, factor that decides the sustainability of life, in a region. Other factors like the temperature, and nature of soil also matter. But water is, one of the major resources which determine, life on land., , the water in rivers can be affected is when water, is released from dams. The water inside the, deep reservoir would be colder than the water, at the surface which gets heated by the Sun., All this can affect the life-forms that are, found in these water bodies in various ways., It can encourage the growth of some life-forms, and harm some other life-forms. This affects, the balance between various organisms which, had been established in that system. So we, use the term water-pollution to cover the, following effects:, 1. The addition of undesirable, substances to water -bodies. These, substances could be the fertilisers and, pesticides used in farming or they, could be poisonous substances, like, mercury salts which are used by, paper-industries. These could also be, disease-causing organisms, like the, bacteria which cause cholera., 2. The removal of desirable substances, from water-bodies. Dissolved oxygen, is used by the animals and plants that, live in water. Any change that reduces, the amount of this dissolved oxygen, would adversely affect these aquatic, organisms. Other nutrients could also, be depleted from the water bodies., 3. A change in temperature. Aquatic, organisms are used to a certain range, of temperature in the water -body, where they live, and a sudden marked, change in this temperature would be, dangerous for them or affect their, breeding. The eggs and larvae of, various animals are particularly, susceptible to temperature changes., , 14.2.1 WATER POLLUTION, Water dissolves the fertilisers and pesticides, that we use on our farms. So some percentage, of these substances are washed into the water, bodies. Sewage from our towns and cities and, the waste from factories are also dumped into, rivers or lakes. Specific industries also use, water for cooling in various operations and, later return this hot water to water-bodies., Another manner in which the temperature of, , Q, , uestions, 1. Why do organisms need water?, 2. What is the major source of fresh, water in the city/town/village, where you live?, 3. Do you know of any activity, which may be polluting this water, source?, , SCIENCE, , 194, , 2019-2020

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14.3 Mineral Riches in the Soil, Soil is an important resource that decides the, diversity of life in an area. But what is the, soil and how is it formed? The outermost layer, of our Earth is called the crust and the, minerals found in this layer supply a variety, of nutrients to life-forms. But these minerals, will not be available to the organisms if the, minerals are bound up in huge rocks. Over, long periods of time, thousands and millions, of years, the rocks at or near the surface of, the Earth are broken down by various, physical, chemical and some biological, processes. The end product of this breaking, down is the fine particles of soil. But what, are the factors or processes that make soil?, •, The Sun: The Sun heats up rocks, during the day so that they expand., At night, these rocks cool down and, contract. Since all parts of the rock, do not expand and contract at the, same rate, this results in the, formation of cracks and ultimately the, huge rocks break up into smaller, pieces., •, Water: Water helps in the formation, of soil in two ways. One, water could, get into the cracks in the rocks formed, due to uneven heating by the Sun. If, this water later freezes, it would cause, the cracks to widen. Can you think, why this should be so? Two, flowing, water wears away even hard rock over, long periods of time. Fast flowing water, often carries big and small particles, of rock downstream. These rocks rub, against other rocks and the resultant, abrasion causes the rocks to wear, down into smaller and smaller, particles. The water then takes these, particles along with it and deposits it, further down its path. Soil is thus, found in places far away from its, parent-rock., •, Wind: In a process similar to the way, in which water rubs against rocks and, wears them down, strong winds also, erode rocks down. The wind also, , •, , carries sand from one place to the, other like water does., Living organisms also influence the, formation of soil. The lichen that we, read about earlier, also grows on the, surface of rocks. While growing, they, release certain substances that cause, the rock surface to powder down and, form a thin layer of soil. Other small, plants like moss, are able to grow on, this surface now and they cause the, rock to break up further. The roots of, big trees sometimes go into cracks in, the rocks and as the roots grow bigger,, the crack is forced bigger., , Activity ____________14.10, •, , •, •, •, •, , Take some soil and put it into a beaker, containing water. The water should be, at least five times the amount of soil, taken. Stir the soil and water vigorously, and allow the soil to settle down., Observe after some time., Is the soil at the bottom of the beaker, homogenous or have layers formed?, If layers have formed, how is one layer, different from another?, Is there anything floating on the, surface of the water?, Do you think some substances would, have dissolved in the water? How would, you check?, , As you have seen, soil is a mixture. It, contains small particles of rock (of different, sizes). It also contains bits of decayed living, organisms which is called humus. In addition,, soil also contains various forms of, microscopic life. The type of soil is decided, by the average size of particles found in it, and the quality of the soil is decided by the, amount of humus and the microscopic, organisms found in it. Humus is a major, factor in deciding the soil structure because, it causes the soil to become more porous and, allows water and air to penetrate deep, underground. The mineral nutrients that are, found in a particular soil depends on the, rocks it was formed from. The nutrient, content of a soil, the amount of humus, present in it and the depth of the soil are, , NATURAL RESOURCES, , 195, , 2019-2020

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some of the factors that decide which plants, will thrive on that soil. Thus, the topmost, layer of the soil that contains humus and, living organisms in addition to the soil, particles is called the topsoil. The quality of, the topsoil is an important factor that decides, biodiversity in that area., Modern farming practices involve the use, of large amounts of fertilizers and pesticides., Use of these substances over long periods of, time can destroy the soil structure by killing, the soil micro-organisms that recycle, nutrients in the soil. It also kills the, Earthworms which are instrumental in, making the rich humus. Fertile soil can, quickly be turned barren if sustainable, practices are not followed. Removal of useful, components from the soil and addition of, other substances, which adversely affect the, fertility of the soil and kill the diversity of, organisms that live in it, is called soil pollution., The soil that we see today in one place, has been created over a very long period of, time. However, some of the factors that, created the soil in the first place and brought, the soil to that place may be responsible for, the removal of the soil too. The fine particles, of soil may be carried away by flowing water, or wind. If all the soil gets washed away and, the rocks underneath are exposed, we have, lost a valuable resource because very little, will grow on the rock., , Activity ____________14.11, •, , •, , •, , Take two identical trays and fill them, with soil. Plant mustard or green gram, or paddy in one of the trays and water, both the trays regularly for a few days,, till the first tray is covered by plant, growth. Now, tilt both the trays and fix, them in that position. Make sure that, both the trays are tilted at the same, angle. Pour equal amount of water, gently on both trays such that the water, flows out of the trays (Fig. 14.4)., Study the amount of soil that is carried, out of the trays. Is the amount the, same in both the trays?, Now pour equal amounts of water on, both the trays from a height. Pour three, or four times the amount that you, poured earlier., , •, , •, , Study the amount of soil that is, carried out of the trays now. Is the, amount the same in both the trays?, Is the amount of soil that is carried out, more or less or equal to the amount, washed out earlier?, , Fig. 14.4: Effect of flowing water on the top-soil, , The roots of plants have an important role, in preventing soil erosion. The large-scale, deforestation that is happening all over the, world not only destroys biodiversity, it also, leads to soil erosion. Topsoil that is bare of, vegetation, is likely to be removed very, quickly. And this is accelerated in hilly or, mountainous regions. This process of soil, erosion is very difficult to reverse. Vegetative, cover on the ground has a role to play in the, percolation of water into the deeper, layers too., , Q, , uestions, 1. How is soil formed?, 2. What is soil erosion?, 3. What are the methods of, preventing or reducing soil, erosion?, , 14.4 Biogeochemical Cycles, A constant interaction between the biotic and, abiotic components of the biosphere makes, it a dynamic, but stable system. These, interactions consist of a transfer of matter, and energy between the different components, of the biosphere. Let us look at some, processes involved in the maintenance of the, above balance., SCIENCE, , 196, , 2019-2020

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14.4.1 THE WATER-CYCLE, You have seen how the water evaporates from, the water bodies and subsequent, condensation of this water vapour leads to, rain. But we don’t see the seas and oceans, drying up. So, how is the water returning to, these water bodies? The whole process in, which water evaporates and falls on the land, as rain and later flows back into the sea via, rivers is known as the water-cycle. This cycle, is not as straight-forward and simple as this, statement seems to imply. All of the water, that falls on the land does not immediately, flow back into the sea. Some of it seeps into, the soil and becomes part of the underground, reservoir of fresh-water. Some of this, underground water finds its way to the, surface through springs. Or we bring it to, the surface for our use through wells or tubewells. Water is also used by terrestrial animals, and plants for various life-processes, (Fig. 14.5)., , Fig. 14.5: Water-cycle in nature, , Let us look at another aspect of what, happens to water during the water-cycle. As, you know, water is capable of dissolving a, large number of substances. As water flows, through or over rocks containing soluble, minerals, some of them get dissolved in the, , water. Thus rivers carry many nutrients from, the land to the sea, and these are used by, the marine organisms., , 14.4.2 THE NITROGEN-CYCLE, Nitrogen gas makes up 78% of our, atmosphere and nitrogen is also a part of, many molecules essential to life like proteins,, nucleic acids (DNA and RNA) and some, vitamins. Nitrogen is found in other, biologically important compounds such as, alkaloids and urea too. Nitrogen is thus an, essential nutrient for all life-forms and life, would be simple if all these life-forms could, use the atmospheric nitrogen directly., However, other than a few forms of bacteria,, life-forms are not able to convert the, comparatively inert nitrogen molecule into, forms like nitrates and nitrites which can be, taken up and used to make the required, molecules. These ‘nitrogen-fixing’ bacteria, may be free-living or be associated with some, species of dicot plants. Most commonly, the, nitrogen-fixing bacteria are found in the roots, of legumes (generally the plants which give, us pulses) in special structures called rootnodules. Other than these bacteria, the only, other manner in which the nitrogen molecule, is converted to nitrates and nitrites is by a, physical process. During lightning, the high, temperatures and pressures created in the, air convert nitrogen into oxides of nitrogen., These oxides dissolve in water to give nitric, and nitrous acids and fall on land along with, rain. These are then utilised by various lifeforms., What happens to the nitrogen once it is, converted into forms that can be taken up, and used to make nitrogen-containing, molecules? Plants generally take up nitrates, and nitrites and convert them into amino, acids which are used to make proteins. Some, other biochemical pathways are used to make, the other complex compounds containing, nitrogen. These proteins and other complex, compounds are subsequently consumed by, animals. Once the animal or the plant dies,, other bacteria in the soil convert the various, compounds of nitrogen back into nitrates and, , NATURAL RESOURCES, , 197, , 2019-2020

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Fig.14.6: Nitrogen-cycle in nature, , nitrites. A different type of bacteria converts, the nitrates and nitrites into elemental, nitrogen. Thus, there is a nitrogen-cycle in, nature in which nitrogen passes from its, elemental form in the atmosphere into simple, molecules in the soil and water, which get, converted to more complex molecules in living, beings and back again to the simple nitrogen, molecule in the atmosphere., , 14.4.3 THE CARBON-CYCLE, Carbon is found in various forms on the, Earth. It occurs in the elemental form as, diamonds and graphite. In the combined, state, it is found as carbon dioxide in the, atmosphere, as carbonate and hydrogencarbonate salts in various minerals, while all, life-forms are based on carbon-containing, molecules like proteins, carbohydrates, fats,, , nucleic acids and vitamins. The endoskeletons, and exoskeletons of various animals are also, formed from carbonate salts. Carbon is, incorporated into life-forms through the basic, process of photosynthesis which is performed, in the presence of Sunlight by all life-forms that, contain chlorophyll. This process converts, carbon dioxide from the atmosphere or, dissolved in water into glucose molecules., These glucose molecules are either converted, into other substances or used to provide, energy for the synthesis of other biologically, important molecules (Fig. 14.7)., The utilisation of glucose to provide energy, to living things involves the process of, respiration in which oxygen may or may not, be used to convert glucose back into carbon, dioxide. This carbon dioxide then goes back, into the atmosphere. Another process that, SCIENCE, , 198, , 2019-2020

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Fig. 14.7: Carbon-cycle in nature, , adds to the carbon dioxide in the atmosphere, is the process of combustion where fuels are, burnt to provide energy for various needs like, heating, cooking, transportation and, industrial processes. In fact, the percentage, of carbon dioxide in the atmosphere is said, to have doubled since the industrial, revolution when human beings started, burning fossil fuels on a very large scale., Carbon, like water, is thus cycled repeatedly, through different forms by the various, physical and biological activities., , 14.4.3 (i) THE GREENHOUSE EFFECT, , could be kept warm during the winters in, colder climates. Such enclosures are called, greenhouses. Greenhouses have also lent their, name to an atmospheric phenomenon. Some, gases prevent the escape of heat from the, Earth. An increase in the percentage of such, gases in the atmosphere would cause the, average temperatures to increase world-wide, and this is called the greenhouse effect. Carbon, dioxide is one of the greenhouse gases. An, increase in the carbon dioxide content in the, atmosphere would cause more heat to be, retained by the atmosphere and lead to global, warming., , Recall the reading taken by you under (iii) in, Activity 14.1. Heat is trapped by glass, and, hence the temperature inside a glass, enclosure will be much higher than the, surroundings. This phenomenon was used, to create an enclosure where tropical plants, NATURAL RESOURCES, , Activity ___________ 14.12, •, •, , Find out what the consequences of, global warming would be., Also, find out the names of some other, greenhouse gases., 199, , 2019-2020

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14.4.4 THE OXYGEN-CYCLE, Oxygen is a very abundant element on our, Earth. It is found in the elemental form in, the atmosphere to the extent of 21%. It also, occurs extensively in the combined form in, the Earth’s crust as well as also in the air in, the form of carbon dioxide. In the crust, it is, found as the oxides of most metals and, silicon, and also as carbonate, sulphate,, nitrate and other minerals. It is also an, essential component of most biological, molecules like carbohydrates, proteins,, nucleic acids and fats (or lipids)., But when we talk of the oxygen-cycle, we, are mainly referring to the cycle that, maintains the levels of oxygen in the, atmosphere. Oxygen from the atmosphere is, used up in three processes, namely, combustion, respiration and in the formation, of oxides of nitrogen. Oxygen is returned to, the atmosphere in only one major process,, that is, photosynthesis. And this forms the, broad outline of the oxygen-cycle in nature, (Fig. 14.8)., , bacteria, are poisoned by elemental oxygen., In fact, even the process of nitrogen-fixing by, bacteria does not take place in the presence, of oxygen., , 14.5 Ozone Layer, Elemental oxygen is normally found in the, form of a diatomic molecule. However, in the, upper reaches of the atmosphere, a molecule, containing three atoms of oxygen is found., This would mean a formula of O3 and this is, called ozone. Unlike the normal diatomic, molecule of oxygen, ozone is poisonous and, we are lucky that it is not stable nearer to, the Earth’s surface. But it performs an, essential function where it is found. It absorbs, harmful radiations from the Sun. This, prevents those harmful radiations from, reaching the surface of the Earth where they, may damage many forms of life., Recently it was discovered that this ozone, layer was getting depleted. Various man-made, compounds like CFCs (carbon compounds, having both fluorine and chlorine which are, very stable and not degraded by any biological, process) were found to persist in the, atmosphere. Once they reached the ozone, layer, they would react with the ozone, molecules. This resulted in a reduction of the, ozone layer and recently they have discovered, a hole in the ozone layer above the Antartica., It is difficult to imagine the consequences for, life on Earth if the ozone layer dwindles, further, but many people think that it would, be better not to take chances. These people, advocate working towards stopping all further, damage to the ozone layer., , Fig. 14.8: Oxygen-cycle in nature, , Though we usually think of oxygen as, being necessary to life in the process of, respiration, it might be of interest to you to, learn that some forms of life, especially, , October, 1980, , October, 1985, , October, 1990, , Fig. 14.9: Satellite picture showing the hole (magenta, colour) in the ozone layer over Antartica, SCIENCE, , 200, , 2019-2020

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Activity ____________14.13, •, •, •, , Find out which other molecules are, thought to damage the ozone layer., Newspaper reports often talk about the, hole in the ozone layer., Find out whether the size of this hole, is changing and in what manner, scientists think this would affect life, on Earth (Fig. 14.9)., , Q, , uestions, 1. What are the different states in, which water is found during the, water cycle?, 2. Name two biologically important, compounds that contain both, oxygen and nitrogen., 3. List any three human activities, which would lead to an increase, in the carbon dioxide content of air., 4. What is the greenhouse effect?, 5. What are the two forms of, oxygen found in the atmosphere?, , What, you have, learnt, •, •, •, •, •, , •, •, , Life on Earth depends on resources like soil, water and air,, and energy from the Sun., Uneven heating of air over land and water-bodies causes winds., Evaporation of water from water -bodies and subsequent, condensation give us rain., Rainfall patterns depend on the prevailing wind patterns in an, area., Various nutrients are used again and again in a cyclic fashion., This leads to a certain balance between the various components, of the biosphere., Pollution of air, water and soil affect the quality of life and, harm the biodiversity., We need to conserve our natural resources and use them in a, sustainable manner., , Exercises, 1., 2., 3., 4., , Why is the atmosphere essential for life?, Why is water essential for life?, How are living organisms dependent on the soil? Are organisms, that live in water totally independent of soil as a resource?, You have seen weather reports on television and in newspapers., How do you think we are able to predict the weather?, , NATURAL RESOURCES, , 201, , 2019-2020

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5., , 6., , We know that many human activities lead to increasing, levels of pollution of the air, water-bodies and soil. Do you, think that isolating these activities to specific and limited, areas would help in reducing pollution?, Write a note on how forests influence the quality of our air,, soil and water resources., , SCIENCE, , 202, , 2019-2020

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Chapter 15, IMPROVEMENT IN FOOD RESOURCES, We know that all living organisms need food., Food supplies proteins, carbohydrates, fats,, vitamins and minerals, all of which we require, for body development, growth and health., Both plants and animals are major sources, of food for us. We obtain most of this food, from agriculture and animal husbandry., We read in newspapers that efforts are, always being made to improve production, from agriculture and animal husbandry. Why, is this necessary? Why we cannot make do, with the current levels of production?, India is a very populous country. Our, population is more than one billion people,, and it is still growing. As food for this growing, population, we will soon need more than a, quarter of a billion tonnes of grain every year., This can be done by farming on more land., But India is already intensively cultivated. As, a result, we do not have any major scope for, increasing the area of land under cultivation., Therefore, it is necessary to increase our, production efficiency for both crops and, livestock., Efforts to meet the food demand by, increasing food production have led to some, successes so far. We have had the green, revolution, which contributed to increased, food-grain production. We have also had the, white revolution, which has led to better and, more efficient use as well as availability of milk., However, these revolutions mean that our, natural resources are getting used more, intensively. As a result, there are more, chances of causing damage to our natural, resources to the point of destroying their, balance completely. Therefore, it is important, that we should increase food production, without degrading our environment and, disturbing the balances maintaining it., , Hence, there is a need for sustainable, practices in agriculture and animal, husbandry., Also, simply increasing grain production, for storage in warehouses cannot solve the, problem of malnutrition and hunger. People, should have money to purchase food. Food, security depends on both availability of food, and access to it. The majority of our, population depends on agriculture for their, livelihood. Increasing the incomes of people, working in agriculture is therefore necessary, to combat the problem of hunger. Scientific, management practices should be undertaken, to obtain high yields from far ms. For, sustained livelihood, one should undertake, mixed farming, intercropping, and integrated, farming practices, for example, combine, agriculture with livestock/poultry/fisheries/, bee-keeping., The question thus becomes – how do we, increase the yields of crops and livestock?, , 15.1 Improvement in Crop Yields, Cereals such as wheat, rice, maize, millets, and sorghum provide us carbohydrate for, energy requirement. Pulses like gram (chana),, pea (matar), black gram (urad), green gram, (moong), pigeon pea (arhar), lentil (masoor),, provide us with protein. And oil seeds, including soyabean, ground nut, sesame,, castor, mustard, linseed and sunflower, provide us with necessary fats (Fig. 15.1)., Vegetables, spices and fruits provide a range, of vitamins and minerals in addition to small, amounts of proteins, carbohydrates and fats., In addition to these food crops, fodder crops, like berseem, oats or sudan grass are raised, as food for the livestock., , 2019-2020

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the kharif season from the month of June to, October, and some of the crops are grown in, the winter season, called the rabi season from, November to April. Paddy, soyabean, pigeon, pea, maize, cotton, green gram and black, gram are kharif crops, whereas wheat, gram,, peas, mustard, linseed are rabi crops., In India there has been a four times, increase in the production of food grains from, 1952 to 2010 with only 25% increase in the, cultivable land area. How has this increase, in production been achieved? If we think of, the practices involved in farming, we can see, that we can divide it into three stages. The, first is the choice of seeds for planting. The, second is the nurturing of the crop plants., The third is the protection of the growing and, harvested crops from loss. Thus, the major, groups of activities for improving crop yields, can be classified as:, • Crop variety improvement, • Crop production improvement, • Crop protection management., , 15.1.1 CROP VARIETY IMPROVEMENT, , Fig. 15.1: Different types of crops, , Q, , uestion, 1. What do we get from cereals,, pulses, fruits and vegetables?, , Different crops require different climatic, conditions, temperature and photoperiods for, their growth and completion of their life cycle., Photoperiods are related to the duration of, sunlight. Growth of plants and flowering are, dependent on sunlight. As we all know, plants, manufacture their food in sunlight by the, process of photosynthesis. There are some, crops, which are grown in rainy season, called, , This approach depends on finding a crop, variety that can give a good yield. Varieties or, strains of crops can be selected by breeding, for various useful characteristics such as, disease resistance, response to fertilisers,, product quality and high yields. One way of, incorporating desirable characters into crop, varieties is by hybridisation. Hybridisation, refers to crossing between genetically, dissimilar plants. This crossing may be, intervarietal (between different varieties),, interspecific (between two different species of, the same genus) or intergeneric (between, different genera). Another way of improving, the crop is by introducing a gene that would, provide the desired characteristic. This, results in genetically modified crops., For new varieties of crops to be accepted,, it is necessary that the variety produces high, yields under different conditions that are, found in different areas. Farmers would need, to be provided with good quality seeds of a, particular variety, that is, the seeds should, , 204, , SCIENCE, , 2019-2020

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all be of the same variety and germinate under, the same conditions., Cultivation practices and crop yield are, related to weather, soil quality and availability, of water. Since weather conditions such as, drought and flood situations are, unpredictable, varieties that can be grown in, diverse climatic conditions are useful., Similarly, varieties tolerant to high soil salinity, have been developed. Some of the factors for, which variety improvement is done are:, •, Higher yield: To increase the, productivity of the crop per acre., •, Improved, quality:, Quality, considerations of crop products vary, from crop to crop. Baking quality is, important in wheat, protein quality in, pulses, oil quality in oilseeds and, preserving quality in fruits and, vegetables., •, Biotic and abiotic resistance: Crops, production can go down due to biotic, (diseases, insects and nematodes) and, abiotic (drought, salinity, water, logging, heat, cold and frost) stresses, under different situations. Varieties, resistant to these stresses can improve, crop production., •, Change in maturity duration: The, shorter the duration of the crop from, sowing to harvesting, the more, economical is the variety. Such short, durations allow far mers to grow, multiple rounds of crops in a year., Short duration also reduces the cost, of crop production. Uniform maturity, makes the harvesting process easy, and reduces losses during harvesting., •, Wider adaptability: Developing, varieties for wider adaptability will, help in stabilising the crop production, under dif ferent envir onmental, conditions. One variety can then be, grown under dif ferent climatic, conditions in different areas., •, Desirable agronomic characteristics:, Tallness and profuse branching are, desirable characters for fodder crops., Dwarfness is desired in cereals, so, IMPROVEMENT, , IN, , that less nutrients are consumed by, these crops. Thus developing varieties, of desired agronomic characters help, give higher productivity., , Q, , uestions, 1. How do biotic and abiotic factors, affect crop production?, 2. What are the desirable agronomic, characteristics, for, crop, improvements?, , 15.1.2 CROP PRODUCTION MANAGEMENT, In India, as in many other agriculture-based, countries, farming ranges from small to very, large farms. Different farmers thus have more, or less land, money and access to information, and technologies. In short, it is the money or, financial conditions that allow farmers to take, up different farming practices and agricultural, technologies. There is a correlation between, higher inputs and yields. Thus, the farmer’s, purchasing capacity for inputs decides, cropping system and production practices., Therefore, production practices can be at, different levels. They include ‘no cost’, production, ‘low cost’ production and ‘high, cost’ production practices., , 15.1.2 (i) NUTRIENT MANAGEMENT, Just as we need food for development, growth, and well-being, plants also require nutrients, for growth. Nutrients are supplied to plants, by air, water and soil. There are several, nutrients which are essential for plants. Air, supplies carbon and oxygen, hydrogen comes, from water, and soil supplies the other thirteen, nutrients to plants. Amongst these, some are, required in large quantities and are therefore, called macro-nutrients. The other nutrients are, used by plants in small quantities and are, therefore called micro-nutrients (Table 15.1)., 205, , FOOD RESOURCES, , 2019-2020

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Table 15.1: Nutrients supplied, by air, water and soil, Source, , Nutrients, , Air, , carbon, oxygen, , Water, , hydrogen, oxygen, , Soil, , (i) Macronutrients:, nitrogen, phosphorus,, potassium, calcium,, magnesium, sulphur, (ii) Micronutrients:, iron, manganese, boron,, zinc,, copper,, molybdenum, chlorine, , Deficiency of these nutrients affects, physiological processes in plants including, reproduction, growth and susceptibility to, diseases. To increase the yield, the soil can be, enriched by supplying these nutrients in the, form of manure and fertilizers., , Q, , uestions, , our environment from excessive use of, fertilizers. Using biological waste material is, also a way of recycling farm waste. Based on, the kind of biological material used, manure, can be classified as:, (i) Compost and vermi-compost: The, process in which farm waste material, like livestock excreta (cow dung etc.),, vegetable waste, animal refuse,, domestic waste, sewage waste, straw,, eradicated weeds etc. is decomposed, in pits is known as composting. The, compost is rich in organic matter and, nutrients. Compost is also prepared, by using earthworms to hasten the, process of decomposition of plant and, animal refuse. This is called vermicompost., (ii) Green manure: Prior to the sowing of, the crop seeds, some plants like sun, hemp or guar are grown and then, mulched by ploughing them into the, soil. These green plants thus turn into, green manure which helps in, enriching the soil in nitrogen and, phosphorus., , FERTILIZERS, , 1. What are macro-nutrients and, why are they called macronutrients?, 2. How do plants get nutrients?, , MANURE, Manure contains large quantities of organic, matter and also supplies small quantities of, nutrients to the soil. Manure is prepared by, the decomposition of animal excreta and plant, waste. Manure helps in enriching soil with, nutrients and organic matter and increasing, soil fertility. The bulk of organic matter in, manure helps in improving the soil structure., This involves increasing the water holding, capacity in sandy soils. In clayey soils, the, large quantities of organic matter help in, drainage and in avoiding water logging., In using manure we use biological waste, material, which is advantageous in protecting, , Fertilizers are commercially produced plant, nutrients. Fertilizers supply nitrogen,, phosphorus and potassium. They are used, to ensure good vegetative growth (leaves,, branches and flowers), giving rise to healthy, plants. Fertilizers are a factor in the higher, yields of high-cost farming., Fertilizers should be applied carefully in, terms of proper dose, time, and observing preand post-application precautions for their, complete utilisation. For example, sometimes, fertilizers get washed away due to excessive, irrigation and are not fully absorbed by the, plants. This excess fertilizer then leads to, water pollution., Also, as we have seen in the previous, chapter, continuous use of fertilizers in an, area can destroy soil fertility because the, organic matter in the soil is not replenished, and micro-organisms in the soil are harmed, by the fertilizers used. Short-term benefits of, using fertilizers and long-term benefits of, , 206, , SCIENCE, , 2019-2020

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using manure for maintaining soil fertility have, to be considered while aiming for optimum, yields in crop production., , Q, , uestion, 1. Compare the use of manure and, fertilizers in maintaining soil, fertility., , Organic farming is a farming system with, minimal or no use of chemicals as fertilizers,, herbicides, pesticides etc. and with a, maximum input of organic manures, recycled, farm-wastes (straw and livestock excreta), use, of bio-agents such as culture of blue green, algae in preparation of biofertilizers, neem, leaves or turmeric specifically in grain storage, as bio-pesticides, with healthy cropping, systems [mixed cropping, inter-cropping and, crop rotation as discussed below in, 15.1.2.(iii)]. These cropping systems are, beneficial in insect, pest and wheat control, besides providing nutrients., , 15.1.2 (ii) IRRIGATION, , More to know, , Most agriculture in India is rain-fed, that is,, the success of crops in most areas is, dependent on timely monsoons and sufficient, rainfall spread through most of the growing, season. Hence, poor monsoons cause crop, failure. Ensuring that the crops get water at, the right stages during their growing season, can increase the expected yields of any crop., Therefore, many measures are used to bring, more and more agricultural land under, irrigation., Droughts occur because of scarcity or, irregular distribution of rains. Drought, poses a threat to rain-fed farming, areas, where farmers do not use, irrigation for crop production and, depend only on rain. Light soils have, less water retention capacity. In areas, with light soils, crops get adversely, affected by drought conditions., Scientists have developed some crop, varieties which can tolerate drought, conditions., , IMPROVEMENT, , IN, , India has a wide variety of water resources, and a highly varied climate. Under such, conditions, several different kinds of irrigation, systems are adopted to supply water to, agricultural lands depending on the kinds of, water resources available. These include wells,, canals, rivers and tanks., •, Wells: There are two types of wells,, namely dug wells and tube wells. In a, dug well, water is collected from water, bearing strata. Tube wells can tap, water from the deeper strata. From, these wells, water is lifted by pumps, for irrigation., •, Canals: This is usually an elaborate, and extensive irrigation system. In this, system canals receive water from one, or more reservoirs or from rivers. The, main canal is divided into branch, canals having further distributaries to, irrigate fields., •, River Lift Systems: In areas where, canal flow is insufficient or irregular, due to inadequate reservoir release,, the lift system is more rational. Water, is directly drawn from the rivers for, supplementing irrigation in areas, close to rivers., •, Tanks: These are small storage, reservoirs, which intercept and store, the run-off of smaller catchment, areas., Fresh initiatives for increasing the water, available for agriculture include rainwater, harvesting and watershed management. This, involves building small check-dams which, lead to an increase in ground water levels., The check-dams stop the rainwater from, flowing away and also reduce soil erosion., , 15.1.2 (iii) CROPPING PATTERNS, Different ways of growing crops can be used, to give maximum benefit., Mixed cropping is growing two or more, crops simultaneously on the same piece of, land, for example, wheat + gram, or wheat +, mustard, or groundnut + sunflower. This, reduces risk and gives some insurance, against failure of one of the crops., 207, , FOOD RESOURCES, , 2019-2020

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Inter-cropping is growing two or more, crops simultaneously on the same field in a, definite pattern (Fig.15.2). A few rows of one, crop alternate with a few rows of a second, crop, for example, soyabean + maize, or finger, millet (bajra) + cowpea (lobia). The crops are, selected such that their nutrient, requirements are different. This ensures, maximum utilisation of the nutrients, supplied, and also prevents pests and, diseases from spreading to all the plants, belonging to one crop in a field. This way,, both crops can give better returns., , Fig. 15.2 : Intercropping, , The growing of different crops on a piece, of land in a pre-planned succession is known, as crop rotation. Depending upon the, duration, crop rotation is done for different, crop combinations. The availability of moisture, and irrigation facilities decide the choice of the, crop to be cultivated after one harvest. If crop, rotation is done properly then two or three, crops can be grown in a year with good, harvests., , 15.1.3 CROP PROTECTION MANAGEMENT, Field crops are infested by a large number of, weeds, insect pests and diseases. If weeds and, pests are not controlled at the appropriate, time then they can damage the crops so much, that most of the crop is lost., Weeds are unwanted plants in the, cultivated field, for example, Xanthium, , (gokhroo), Parthenium (gajar ghas),, Cyperinus rotundus (motha). They compete, for food, space and light. Weeds take up, nutrients and reduce the growth of the crop., Therefore, removal of weeds from cultivated, fields during the early stages of crop growth is, essential for a good harvest., Generally insect pests attack the plants, in three ways: (i) they cut the root, stem and, leaf, (ii) they suck the cell sap from various, parts of the plant, and (iii) they bore into stem, and fruits. They thus affect the health of the, crop and reduce yields., Diseases in plants are caused by, pathogens such as bacteria, fungi and viruses., These pathogens can be present in and, transmitted through the soil, water and air., Weeds, insects and diseases can be, controlled by various methods. One of the, most commonly used methods is the use of, pesticides, which include herbicides,, insecticides and fungicides. These chemicals, are sprayed on crop plants or used for treating, seeds and soil. However, excessive use of, these chemicals creates problems, since they, can be poisonous to many plant and animal, species and cause environmental pollution., Weed control methods also include, mechanical removal. Preventive methods, such as proper seed bed preparation, timely, sowing of crops, intercropping and crop, rotation also help in weed control. Some other, preventive measures against pests are the use, of resistant varieties, and summer ploughing,, in which fields are ploughed deep in summers, to destroy weeds and pests., , Q, , 208, , uestion, 1. Which of the following conditions, will give the most benefits? Why?, (a) Farmers use high-quality, seeds, do not adopt, irrigation or use fertilizers., (b) Far mers use ordinary, seeds, adopt irrigation and, use fertilizer., (c) Farmers use quality seeds,, adopt irrigation, use, fertilizer and use crop, protection measures., SCIENCE, , 2019-2020

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Table 15.2: Nutritional values of animal products, Animal, Products, , Per cent (%) Nutrients, Fat, , Protein, , Sugar, , Minerals, , Water, , 3.60, , 4.00, , 4.50, , 0.70, , 87.20, , B1, B2, B12, D, E, , Egg, , 12.00, , 13.00, , *, , 1.00, , 74.00, , B2, D, , Meat, , 3.60, , 21.10, , *, , 1.10, , 74.20, , B2, B12, , Fish, , 2.50, , 19.00, , *, , 1.30, , 77.20, , Niacin, D, A, , Milk (Cow), , Vitamins, , *Present in very small amounts, , Activity _____________ 15.1, •, , Visit a nearby garden/agricultural, field and make a list of the weeds and, the flowers/crops found in the area., Also, make a list of insect pests, if, any, infesting the flowers/crops., , STORAGE OF, , uestions, 1. Why should preventive measures, and biological control methods be, preferred for protecting crops?, 2. What factors may be responsible, for losses of grains during, storage?, , IMPROVEMENT, , IN, , •, , Collect grains/seeds of cereals, pulses, and oil seeds and gather information, about the seasons in which they are, sown and harvested., , 15.2 Animal Husbandry, , GRAINS, , Storage losses in agricultural produce can be, very high. Factors responsible for such losses, are biotic— insects, rodents, fungi, mites and, bacteria, and abiotic— inappropriate moisture, and temperatures in the place of storage. These, factors cause degradation in quality, loss in, weight, poor germinability, discolouration of, produce, all leading to poor marketability., These factors can be controlled by proper, treatment and by systematic management of, warehouses., Preventive and control measures are used, before grains are stored for future use. They, include strict cleaning of the produce before, storage, proper drying of the produce first in, sunlight and then in shade, and fumigation, using chemicals that can kill pests., , Q, , Activity _____________ 15.2, , Animal husbandry is the scientific, management of animal livestock. It includes, various aspects such as feeding, breeding and, disease control. Animal-based farming, includes cattle, goat, sheep, poultry and fish, farming. As the population increases and as, living standards increase, the demand for, milk, eggs and meat is also going up. Also,, the growing awareness of the need for, humane treatment of livestock has brought, in new limitations in livestock farming. Thus,, livestock production also needs to be improved., , 15.2.1 CATTLE, , FARMING, , Cattle husbandry is done for two purposes—, milk and draught labour for agricultural work, such as tilling, irrigation and carting. Indian, cattle belong to two different species, Bos, indicus, cows, and Bos bubalis, buffaloes., Milk-producing females are called milch, animals (dairy animals), while the ones used, for farm labour are called draught animals., Milk production depends, to some extent,, on the duration of the lactation period,, meaning the period of milk production after, 209, , FOOD RESOURCES, , 2019-2020

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Fig. 15.3: Indigenous milch breed of cattle, , the birth of a calf. So, milk production can be, increased by increasing the lactation period., Exotic or foreign breeds (for example, Jersey,, Brown Swiss) are selected for long lactation, periods, while local breeds (for example, Red, Sindhi, Sahiwal) show excellent resistance to, diseases. The two can be cross-bred to get, animals with both the desired qualities., , Q, , uestion, 1. Which method is commonly used, for improving cattle breeds and, why?, , 15.2.2 POULTRY FARMING, , Activity _____________ 15.3, •, , loose hair. They should be sheltered under, well-ventilated roofed sheds that protect them, from rain, heat and cold. The floor of the cattle, shed needs to be sloping so as to stay dry, and to facilitate cleaning., The food requirements of dairy animals, are of two types: (a) maintenance, requirement, which is the food required to, support the animal to live a healthy life, and, (b) milk producing requirement, which is the, type of food required during the lactation, period. Animal feed includes: (a) roughage,, which is largely fibre, and (b) concentrates,, which are low in fibre and contain relatively, high levels of proteins and other nutrients., Cattle need balanced rations containing all, nutrients in proportionate amounts. Besides, such nutritious food material, certain feed, additives containing micronutrients promote, the health and milk output of dairy animals., Cattle suffer from a number of diseases., The diseases, besides causing death, reduce, milk production. A healthy animal feeds, regularly and has a normal posture. The, parasites of cattle may be both external, parasites and internal parasites. The external, parasites live on the skin and mainly cause, skin diseases. The internal parasites like, worms, affect stomach and intestine while, flukes damage the liver. Infectious diseases, are also caused by bacteria and viruses., Vaccinations are given to farm animals, against many major viral and bacterial, diseases., , Visit a livestock farm. Note the, following:, (1) Number of cattle and number of, different breeds., (2) The amount of daily milk production, from the different breeds., , Proper cleaning and shelter facilities for, cows and buffaloes are required for humane, farming, for the health of the animals and, for production of clean milk as well. Animals, require regular brushing to remove dirt and, , Poultry farming is undertaken to raise, domestic fowl for egg production and chicken, meat. Therefore, improved poultry breeds are, developed and farmed to produce layers for, eggs and broilers for meat., The cross-breeding programmes between, Indian (indigenous, for example, Aseel) and, foreign (exotic, for example, Leghorn) breeds, for variety improvement are focused on to, develop new varieties for the following, desirable traits—, (i) number and quality of chicks;, , 210, , SCIENCE, , 2019-2020

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(ii) dwarf broiler parent for commercial, chick production;, (iii) summer adaptation capacity/, tolerance to high temperature;, (iv) low maintenance requirements;, (v) reduction in the size of the egg-laying, bird with ability to utilise more, fibrous cheaper diets formulated, using agricultural by-products., , Aseel, , Q, , Leghorn, Fig. 15.4, , Q, , uestion, 1. Discuss the implications of the, following statement:, “It is interesting to note that, poultry is India’s most efficient, converter of low fibre food stuff, (which is unfit for human, consumption) into highly, nutritious animal protein food.”, , EGG AND BROILER PRODUCTION, Broiler chickens are fed with vitamin-rich, supplementary feed for good growth rate and, better feed efficiency. Care is taken to avoid, mortality and to maintain feathering and, carcass quality. They are produced as broilers, and sent to market for meat purposes., For good production of poultry birds, good, management practices are important. These, include maintenance of temperature and, hygienic conditions in housing and poultry, feed, as well as prevention and control of, diseases and pests., The, housing,, nutritional, and, environmental requirements of broilers are, somewhat different from those of egg layers., IMPROVEMENT, , IN, , The ration (daily food requirement) for broilers, is protein rich with adequate fat. The level of, vitamins A and K is kept high in the poultry, feeds., Poultry fowl suffer from a number of, diseases caused by virus, bacteria, fungi,, parasites, as well as from nutritional, deficiencies. These necessitate proper, cleaning, sanitation, and spraying of, disinfectants at regular intervals. Appropriate, vaccination can prevent the occurrence of, infectious diseases and reduce loss of poultry, during an outbreak of disease., , uestions, 1. What management practices are, common in dairy and poultry, farming?, 2. What are the differences between, broilers and layers and in their, management?, , Activity _____________ 15.4, •, , Visit a local poultry farm. Observe types, of breeds and note the type of ration,, housing and lighting facilities given to, them. Identify the growers, layers and, broilers., , 15.2.3 FISH PRODUCTION, Fish is a cheap source of animal protein for, our food. Fish production includes the finned, true fish as well as shellfish such as prawns, and molluscs. There are two ways of obtaining, fish. One is from natural resources, which is, called capture fishing. The other way is by, fish farming, which is called culture fishery., The water source of the fish can be either, seawater or fresh water, such as in rivers and, ponds. Fishing can thus be done both by, capture and culture of fish in marine and, freshwater ecosystems., , 15.2.3 (i) MARINE FISHERIES, India’s marine fishery resources include, 7500 km of coastline and the deep seas, 211, , FOOD RESOURCES, , 2019-2020

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beyond it. Popular marine fish varieties, include pomphret, mackerel, tuna, sardines,, and Bombay duck. Marine fish are caught, using many kinds of fishing nets from fishing, boats. Yields are increased by locating large, schools of fish in the open sea using satellites, and echo-sounders., Some marine fish of high economic value, are also farmed in seawater. This includes, finned fishes like mullets, bhetki, and pearl, spots, shellfish such as prawns (Fig. 15.5),, mussels and oysters as well as seaweed., Oysters are also cultivated for the pearls, they make., , In such a system, a combination of five or, six fish species is used in a single fishpond., These species are selected so that they do, not compete for food among them having, different types of food habits. As a result, the, food available in all the parts of the pond is, used. As Catlas are surface feeders, Rohus, feed in the middle-zone of the pond, Mrigals, and Common Carps are bottom feeders, and, Grass Carps feed on the weeds, together these, species (Fig. 15.6) can use all the food in the, pond without competing with each other. This, increases the fish yield from the pond., , (a), Macrobrachium rosenbergii, (fresh water), , (b), , Peneaus monodon, (marine), , Fig. 15.5 : Fresh water and marine prawns, (c), , (d), , As marine fish stocks get further depleted,, the demand for more fish can only be met by, such culture fisheries, a practice called, mariculture., , 15.2.3 (ii) INLAND FISHERIES, , (e), , Fresh water resources include canals, ponds,, reservoirs and rivers. Brackish water, resources, where seawater and fresh water, mix together, such as estuaries and lagoons, are also important fish reservoirs. While, capture fishing is also done in such inland, water bodies, the yield is not high. Most fish, production from these resources is through, aquaculture., Fish culture is sometimes done in, combination with a rice crop, so that fish are, grown in the water in the paddy field. More, intensive fish farming can be done in, composite fish culture systems. Both local, and imported fish species are used in such, systems., , (f), Fig. 15.6: (a) Catla (b) Silver carp (c) Rohu (d) Grass, Carp (e) Mrigal (f) Common Carp, , One problem with such composite fish, culture is that many of these fish breed only, during monsoon. Even if fish seed is collected, from the wild, it can be mixed with that of, other species as well. So, a major problem in, fish farming is the lack of availability of goodquality seed. To overcome this problem, ways, have now been worked out to breed these fish, in ponds using hormonal stimulation. This, has ensured the supply of pure fish seed in, desired quantities., , 212, , SCIENCE, , 2019-2020

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Q, , uestions, 1. How are fish obtained?, 2. What are the advantages of, composite fish culture?, , Activity _____________ 15.5, •, , Visit a fish farm in fish breeding, season and note the following:, (1) Varieties of fish in fish farm, (2) Types of ponds, (3) Feed ingredients used, (4) Production capacity of the farm, If there are no fish farms close to your, locality, gather the above information from, Internet, by referring books or talking to, people who are engaged in fishery., , 15.2.4 BEE-KEEPING, Honey is widely used and therefore beekeeping for making honey has become an, agricultural enterprise. Since bee-keeping, needs low investments, farmers use it as an, additional income generating activity. In, addition to honey, the beehives are a source, of wax which is used in various medicinal, preparations., The local varieties of bees used for, commercial honey production are Apis cerana, indica, commonly known as the Indian bee,, A. dorsata, the rock bee and A. florae, the little, bee. An Italian bee variety, A. mellifera, has, also been brought in to increase yield of honey., , (a), , (b), , Fig. 15.7: (a) Arrangement of beehive in an apiary, (b) honey extractor, , This is the variety commonly used for, commercial honey production., The Italian bees have high honey collection, capacity. They sting somewhat less. They stay, in a given beehive for long periods, and breed, very well. For commercial honey production,, bee farms or apiaries are established., The value or quality of honey depends, upon the pasturage, or the flowers available, to the bees for nectar and pollen collection., In addition to adequate quantity of pasturage,, the kind of flowers available will determine, the taste of the honey., , Q, , uestions, 1. What are the desirable, characters of bee varieties, suitable for honey production?, 2. What is pasturage and how is it, related to honey production?, , What, you have, learnt, •, , •, , IMPROVEMENT, , IN, , There are several nutrients essential for crops. Of these,, some are required in large quantities and are known as, macro-nutrients whereas rest of the nutrients are required, in small quantities and are known as micro-nutrients., Manure and fertilizers are the main sources of nutrient, supply to crops., 213, , FOOD RESOURCES, , 2019-2020

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•, , •, •, •, •, •, , •, , •, , •, •, •, •, •, •, , Organic farming is a farming system with minimal or no, use of chemicals as fertilizers, herbicides, pesticides etc., and with a maximum input of organic manures, recyled, farm wastes, and bio-agents, with healthy cropping systems., Mixed farming is a system of farming on a particular farm, which includes crop production, raising of livestock etc., Mixed cropping is growing of two or more crops simultaneously, on the same piece of land., Growing two or more crops in definite row patterns is known, as inter-cropping., The growing of different crops on a piece of land in pre-planned, succession is called crop rotation., Varietal improvement is required for higher yield, good quality,, biotic and abiotic resistance, shortening the maturity duration,, wider adaptability and desirable agronomic characteristics., Farm animals require proper care and management such as, shelter, feeding, breeding and disease control. This is called, animal husbandry., Poultry farming is done to raise domestic fowls. Poultry, production includes egg production and broiler production for, poultry meat., To enhance poultry production, cross breeding is done between, Indian and exotic breeds for variety improvement., Fish may be obtained from marine resources as well as inland, resources., To increase production of fish, they can be cultured in marine, and inland ecosystems., Marine fish capture is done by fishing nets guided by echosounders and satellites., Composite fish culture system is commonly used for fish, farming., Bee-keeping is done to get honey and wax., , Exercises, 1., 2., 3., 4., , Explain any one method of crop production which ensures, high yield., Why are manure and fertilizers used in fields?, What are the advantages of inter-cropping and crop rotation?, What is genetic manipulation? How is it useful in agricultural, practices?, , 214, , SCIENCE, , 2019-2020

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5., 6., 7., 8., 9., , IMPROVEMENT, , IN, , How do storage grain losses occur?, How do good animal husbandry practices benefit farmers?, What are the benefits of cattle farming?, For increasing production, what is common in poultry, fisheries, and bee-keeping?, How do you differentiate between capture fishing, mariculture, and aquaculture?, , 215, , FOOD RESOURCES, , 2019-2020

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, Chapter 3, 4., , (a), , MgCl2, , (b), , CaO, , (c), (d), , Cu (NO3)2, AlCl3, , (e), (a), (b), , CaCO3, Calcium, oxygen, Hydrogen, bromine, , (c), (d), (a), , Sodium, hydrogen, carbon and oxygen, Potassium, sulphur and oxygen, 26 g, , (b), (c), , 256 g, 124 g, , 7., , (d), (e), (a), , 36.5 g, 63 g, 14 g, , 8., , (b), (c), (a), , 108 g, 1260 g, 0.375 mole, , 9., , (b), (c), (a), , 1.11 mole, 0.5 mole, 3.2 g, , 5., , 6., , 10., , (b) 9.0 g, 3.76 × 1022 molecules, , 11., , 6.022 × 1020 ions, , Chapter 4, 10., , 80.006, , 11., , 16, 8 ×, , 12., , =90% ,, , 18, 8 ×, , = 10%, , Valency = 1, Name of the element is lithium,, , 13., 14., , Mass number of X =12, Y=14, Relationship is Isotope., (a) F, (b) F, (c) T, (d) F, , 15., 16., , (a) , (a) ×, , (b) ×, (b) ×, , (c) ×, (c) , , (d) ×, (d) ×, , 216, , SCIENCE, , 2019-2020

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17., 18., 19., , (b) , (b) ×, , (a) ×, (a) ×, , (c) ×, (c) ×, , (d) ×, (d) , , Atomic, Number, , Mass, Number, , Number, of, Neutrons, , Number, of, Protons, , Number Name of the, of, Atomic, Electrons, Species, , 9, , 19, , 10, , 9, , 9, , Fluorine, , 16, , 32, , 16, , 16, , 16, , Sulphur, , 12, , 24, , 12, , 12, , 12, , Magnesium, , 01, , 2, , 01, , 1, , 01, , Deuterium, , 01, , 1, , 0, , 1, , 0, , Protium, , Chapter 8, 1., 2., , (a), (a), , distance = 2200 m; displacement = 200 m., –1, average speed = average velocity = 2.00 m s, , 3., 4., , (b) average speed = 1.90 m s ; average velocity = 0.952 m s, –1, average speed = 24 km h, distance travelled = 96 m, , –1, , 7., 10., , –1, , –1, , velocity = 20 m s ; time = 2 s, –1, speed = 3.07 km s, , Chapter 9, 4., 5., , c, –2, 2 m s , 14000 N, , 6., 7., , – 4N, (a) 35000 N, , 8., 9., , (b) 1.944 m s, 2550 N in a direction opposite to the motion of the vehicle, d, , –2, , 10., 11., 13., , 200 N, –1, 0ms, –1, 3 kg m s, , 14., 15., , 2.25 m; 50 N, –1, –1, –1, 10 kg m s ; 10 kg m s ; 5/3 m s, , 16., 18., A2., , 500 kg m s ; 800 kg m s ; 50 N, –1, 40 kg m s, 240 N, , A3., A4., , 2500 N, –2, –1, 5 m s ; 2400 kg m s ; 6000 N, , –1, , –1, , 217, , ANSWERS, , 2019-2020

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Chapter 10, 3., 12., , 9.8 N, Weight on earth is 98 N and on moon is 16.3 N., , 13., 14., 15., , Maximum height is 122.5 m and total time is 5 s + 5 s = 10 s., 19.6 m/s, Maximum height = 80 m, Net displacement = 0, Total distance covered = 160 m., , 16., 17., 18., 21., , Gravitational force = 3.56 × 1022 N., 4 s, 80 m from the top., Initial velocity = 29.4 m s–1, height = 44.1 m. After 4 s the ball will be at a distance of 4.9 m, from the top or 39.2 m from the bottom., The substance will sink., , 22., , The packet will sink. The mass of water displaced is 350 g., , Chapter 11, 2., , Zero, , 4., 5., 9., , 210 J, Zero, 9 × 108 J, , 10., 11., 14., , 2000 J, 1000 J, Zero, 15 kWh (Unit), , 17., 18., , 208333.3 J, (i) Zero, , 20., , (ii) Positive, (iii) Negative, 20 kWh, , Chapter 12, 7., , 17.2 m, 0.0172 m, , 8., , 18.55, , 9., 13., 14., , 6000, 11.47 s, 22,600 Hz, , 20., , 1450 ms-1, , 218, , SCIENCE, , 2019-2020